DCF77 / TDF receiver - critique my first design - Page 1
Forum section: electronics / rf-microwave

author: faust1002
author_rank: Contributor
timestamp: February 16, 2020, 06:43:15 pm
content: Hello guys,I am an embedded software developer working on various different radio access technologies, like LTE and NR. After several years in this industry I decided to design my own piece of hardware. I wanted it to be related to RF and at the same point simple enough for a beginner.Short theory of operation1. I have no idea what input signal level expect. Furthermore, based on what I found, this may vary depending on a location and a direction of an antenna. Therefore I don't know how much gain I need. That is why I decided to use a digital potentiometer (U5) to set gain digitally. Furthermore, an operational amplifier (U4A) acts as a differential amplifier to set a virtual ground for the gain stage (U4B).2. I want to use single voltage rail, therefore I must use an inverting configuration (at least this is my understanding).3. I initially thought about designing DCF77 receiver, but then I learnt that there are other similar standards, like TDF for instance, operating in a similar frequency bands. Therefore finally I decided to create an analog fronted, which is capable of receiving a wideband signal between 50kHz - 200kHz (Butterworth filter, U4C and U4D), roughly speaking. Then I will do additional digital filtering to get my signal of interest.My requirements and extra comments.1. I am a software developer, not hardware designer. This means that I don't necessary know what I am doing2. I haven't simulated my design yet. I want to know if I go in the right direction and what alternatives I have before I will put more time in this project.3. I have no soldering equipment at home. Therefore I decided that I will get my project assembled by JLC PCB. This means, I was limited by component available in their library. On the other hand, I could use SMT component, which would be a pain in the neck to solder on my own.4. I also didn't want to create my own component models. Therefore my second requirement was an availability of models in KiCAD or in Ultra Librarian.5. I selected STM8L151K4T6 for a couple of reasons:- it seems simply enough for beginning- maybe at one day I decide to create a battery powered version of my project. Then low power micro would be useful.- I have ST-Link available6. I know that it is possible to receive DCF77 signal using a single diode and BJT, but I wanted something more interesting.7. This project is made purely for fun.I really curious to get your feedback. I will appreciate it.Kind regards

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2020, 07:46:22 pm
content: Unless one is very close to the station the signal level is quite low. So the LM324 is not a suitable amplifier, at least not for the initial, noise sensitive part.One can do a lot of the reception in software, but it still need some initial low noise amplification and filtering. The first filtering is usually a resonant antenna (ferrite rod or a larger area air coil), followed by a low noise amplifier (e.g. a discrete JFET) directly at the antenna.The STM8 µC may not have the power to really do the full software defined radio at the high frequency. For this is would be more like STM32 level with a better ADC and more power.It may still work with an external mixer to make it s super-het receiver. A simple straight through receiver can be tricky, as the gain needed is quite high and there is a good chance to get oscillation from parasitic feedback from the amplifier to the antenna. At least it would need some experience. One should still get a soldering iron to do some soldering yourself. At least the antenna and also some teaks should really be done locally. It would be hard to make is work without a scope.I don't think this is a good project for an absolute beginner.The beginner level would be more like using a ready made receiver module and do the decoding part. To get the time from the pulse pattern.A simpler slightly RF like project could be something like an ultra-sonic sender and receive circuit. This could be either continuous or pulsed (e.g. echo detection for a distance measurement).

author: faust1002
author_rank: Contributor
timestamp: February 16, 2020, 08:00:35 pm
content: Thanks for your reply. I want avoid a precisely tuned tank circuit, because I want my frontend to cover more than more one standard.I can replace LM324 with something based on JFETs, for instance some variation of TL074. Should be good enough. What do you think?I would still claim that STM8 micro has enough horse power for this project. First of all, we are talking here about a carrier frequency in order of kHz. Secondly, the internal built-in clock goes up to 16MHz. Finally, it has 12-bit, 1Msps ADC. Plus reasonable amount of FLASH and SRAM. What else do you need to process low-frequency signals?Kind regards

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2020, 09:50:32 pm
content: The STM8 may be OK, but it can still need quite a few operations to do the signal processing. I don't know the STM8's very well, but with the small ones, one may run out of RAM (if FFT us used) and calculations with 16 bit or 24 bit numbers get quite a bit slower with a 8 bit µC. The SW may need things like look-up tables for sin/cos and these may take up quite some memory. Chances are it can work, but there could be limitations.One would hardly get around the tuned antenna to get good sensitivity. A good antenna is the best amplification and keeping the BW low from the beginning reduces problems with the dynamic range and strong local interfering signals (e.g. SMPS).With only 12 Bit ADC resolution one would likely need quite some analog filtering, as there can be quite a few strong sources of interference, even without the old TV sets in the old times.The TL074 is not really that low in noise - it would still reduce the sensitivity quite a bit, especially if used without a resonant antenna. At least it may be fast enough. It may work with the TL074 if a good tuned antenna is used, but hardly without. Similar a non resonant antenna may just work with a really good amplifier (e.g. good discrete JFET).

author: faust1002
author_rank: Contributor
timestamp: February 17, 2020, 06:21:58 am
content: I am not worried about the digital part. They pay me to do similar things, so I guess I know how to do this correctlyAbout analog filtering - I added a bandpass filter. I suppose this will be enough. Of course I can do something much more complex, but this will not help if intereferences are present in the band of interest.About the resonant antenna, I believe that covering more than one carrier frequency might be possible, but hard. It is correct?Rolling my own discrete, low noise JFET amp is beyond my capabilities now. Therefore I will build the first prototype and see that input levels I will get.

author: AndersJ
author_rank: Frequent Contributor
timestamp: February 17, 2020, 06:44:12 am
content: Some of your resistors are very small.Such as LED serial r’s and those in the filter.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 17, 2020, 07:03:07 am
content: So you want to receive DCF and the like within 50 ... 150kHz?LM324 is too slow to provide reasonable gain here, TL074 requires a bipolar supply of at least 9V (ballpark) differential between V+ and V-.To get enough signal input to the ADC, you'd have to amplify a 10uV ... 1mV signal to a 0.1Vpp to 3.3Vpp signal (your ADC should be able to work within that range), that's a gain somewhere in the range from 1000 ... 10000. From a personal project of mine, I've got good results with a total gain of 3300, split into three amplifier stages. To run the amplifier from a single 3.3V supply, I've used the TLV2372 OpAmp (also available as a quad, TLV2374). For strong signals, I've added a simple limiter using back-to-back diodes. For this particular project, a variable gain wasn't necessary, as the dynamic range of the MCUs ADC was sufficient (your STM8 has only 10 bits, the STM32 I used has 12).If you want to do direct carrier reception, a bandpass filter is a good choice. To suppress noise and interference, it is best placed before the high gain stages, say a tuned ferrite antenna will give best results. Anyway, I've got good results using an untuned antenna too with my project,look it up here for your reference:https://wunderkis.de/dcf-rcvr/

author: faust1002
author_rank: Contributor
timestamp: February 17, 2020, 07:28:28 am
content: @AndersJI want to use green LEDs. Based on my knowledge, they draw about 20mA and their voltage drop is about 2.2V. Therefore I guess that 56Ohms is enough here.Resistors in the bandpass filter were calculated with the assumption that I would use 47nF capacitor. I also believe that they are correct.@capt bullshotI am familiar with your design. I found it while researching my own project. Your website says you used TL072, hence I replaced LM324 with TL074. But TLV2374 seems to be much better choice.Quick question - what are benefits of splitting the gain stage into three steps.Extra footnote - my micro has 12-bit ADC, at least according to its datasheet

author: capt bullshot
author_rank: Super Contributor
timestamp: February 17, 2020, 07:56:05 am
content: I've changed the OpAmp on the way from first perfboard prototype to the final board, just for the reason of a single 3.3V supply.Splitting the gain: It's for BW and DC stability. Your typical TL074 has about 3MHz GBW (gain bandwidth product), so a single 1000x gain stage would have a resulting BW of 3MHz / 1000 -> 3kHz, way too low for this purpose. Having three x15 stages results in a BW of 200kHz per stage, and somewhere above 100kHz for the chain. Also, the AC coupling between the stages helps dealing with the input offset of the amplifiers. A single stage would show 1000x the input offset voltage (in the single volt ballpark here), due to the capacitive coupling only the last stages input offset is of concern in my design (a mere few mV * 15 -> some 10 mV).Didn't look up your micros datasheet, the last STM8 I'd used had a 10bit converter.Edit: There's quite a gotcha when designing filters with OpAmps: Usually one uses some kind of design software to determine the components (at least today, back in the days, it was a bunch of formulas to solve on your own). A good filter designer informs you about the required GBW of the OpAmp, take this for serious. An LM358 or TL072 often doesn't have enough GBW for an analog filter.

author: faust1002
author_rank: Contributor
timestamp: February 17, 2020, 08:09:59 am
content: Thanks for your clarification. I guess I will add another jellybeam opamp to set a virtual ground. Then, I will divide the gain stage - one with fixed gain and the other with variable one.I will update my design in the evening.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2020, 09:13:14 am
content: For the analog part there are mainly 3 difficulties:1) Avoiding interference from strong other signals like a SMPS. Even with a 12 Bit ADC this essentially needs quite some filtering before the ADC.2) keeping the noise down. This is mainly about the first stage. A parallel resonant antenna has a high impedance an is thus reasonable well matched to a FET amplifier. This can be a good OP, and even the TL074 may not be so bad, but there are better ones. A non resonant antenna usually has much lower impedance and would need a lower noise amplifier. Here the Tl074 is likely not sufficient. A resonant antenna really makes things easier as it also provides initial filtering and thus help with the 2 fist points.3) The probably most tricky part is avoiding oscillation due to coupling back the the signal from the amplifier output and supply current to the antenna. Even with better behaved super-het receivers the usual way is to keep the antenna at some distance form the main circuit and only have an initial amplifier directly at the antenna. Using a virtual ground can be tricky in this context.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 17, 2020, 09:46:38 am
content: I've been playing with one of thesehttps://www.amazon.co.uk/60kHz-Atomic-Clock-Radio-Receiver/dp/B01KH3VEGSrather than a discrete design and find that it's extremely susceptible to RFI, which makes me wonder what precautions will need to be taken to prevent noise propagating from the microcontroller to the head-end. Also as I've noted before the C1 variant of the chip used on that module doesn't allow the phase of the input to be tracked, which at least in the case of MSF is an important part of the timing information.A tunable receiver with provision for an external antenna, signal strength meter and phase recovery would /very/ nice to have...MarkMLl

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2020, 10:08:18 am
content: The RFI emissions from the µC should not be so bad, as this is in a different frequency band. The tricky part in a simple single frequency receiver is the last amplifier stage that may produce a magnetic field that can be picked up by the antenna.A kind of software defined receiver should also recover the phase information. The German DCF77 signal also contains some phase modulation part for extra precision and extra information, though not really needed for just the time.

author: chris_leyson
author_rank: Super Contributor
timestamp: February 17, 2020, 11:17:51 am
content: ARM Radio springs to mind.https://www.sdradio.eu/weaksignals/armradio/index.html

author: faust1002
author_rank: Contributor
timestamp: February 18, 2020, 12:19:52 am
content: I updated my design based on your feedback. My changes include- Splitting the gain stage into two parts. The first part has a fixed gain, about 12. The gain of the second part depends on the setting of the digital potentiometer.- Replacing the bandpass filter with a low pass one. I didn't have any mixer anyway, therefore this hardly matters for me. The roll-off frequency is about 170kHz.Your comments are welcome.

author: bsdphk
author_rank: Regular Contributor
timestamp: February 18, 2020, 06:07:24 am
content: I would roll of a bit higher to also get 198kHz inside the reception window.You may find some inspiration in two of my old Loran-C projects:http://phk.freebsd.dk/loran-c/http://phk.freebsd.dk/AducLoran/

author: capt bullshot
author_rank: Super Contributor
timestamp: February 18, 2020, 07:08:41 am
content: LF393 won't work from a single 3.3V supply.The filter circuit around U5 looks buggy to me, you've got R20 and R24 connected to GND from an OpAmps output, this looks wrong.Anyway, better use a real OpAmp symbol for U5 and U3, this makes the schematic more readable.Any capacitors within the filter network that are connected to virtual GND can be connected to real GND for better results.Don't separate GND and AGND, use a bold GND plane for everything.Double check the need for the buffered virtual GND, often this can be replaced by a simple resistive divider and a cap to GND, given that all nodes connected to this virtual point are high impedance (say otherwise unconnected "+" inputs of OpAmps), you can choose an appropriate filter circuit to achieve this. An LM358 as a virtual GND loaded by AC (through the capacitors in the filter network) but not AC bypassed can lead to "interesting" results. LM358 is cheap and easy, but has some pits for the beginner to fall into, so if you can, avoid it.Your VGND0 divider network consists of a 1:1 divider (R8,R9) loaded by another 1:1 divider (R11,R13) - did you take this into calculation? Otherwise the result might surprise you.

author: faust1002
author_rank: Contributor
timestamp: February 18, 2020, 10:36:09 pm
content: In the previous design I used LF253, not LF393. But this doesn't matter anyway, because its minimum voltage supply is 6V according to the datasheet. I used a Digikey parameteric search and didn't check all details. And in the morning I had an ugly surprise.I replaced LF253 with TS462. It seems perfect for this job - it is rail to rail, low noise op amp with a high GBW (12MHz).Could you elaborate a little bit more on what you mean by a single rail op amp? I thought that all op amps are in some shape or form single rail, but you can't go all the way up to the voltage supply.I agree that using op amps symbols for U3 and U5 is way better. It makes schematics not only more readable, but also easier to lay down. However, I was not able to find symbols for some op amps I used. Therefore I imported symbols from Ultra Librarian, which were, shall we say, not the highest quality.In the latest version, the first gain stage has about 35 of gain. Then, the next stage has a variable gain set by U4. Finally, the low pass filter has also about 40dB of gain, hence R13 and R17 are connected to REF. I used Filter Wizard by AD to design this part of the circuit. For time being, I don't want to modify this part of circuit. I believe that breadboarding, measuring it and changing (improving) it if needed is the best way forward. I am a little bit sceptical about simulating it, because I am not sure how accurate results I would get.I didn't fully get the last part of your post. I buffered virtual GND, because it was suggested by Filter Wizard. I agree that it may not be needed. But why LM358 is not a good part here?Also, two voltage dividers are "in parallel", not "in serial". Therefore one doesn't load the other - or maybe I missed something.

author: awallin
author_rank: Frequent Contributor
timestamp: February 19, 2020, 07:07:29 am
content: Quote from: faust1002 on February 16, 2020, 06:43:15 pmIf you want to dive into the software side then get a mini-whip antenna from ebay (30 eur), build a simple bias-T that powers the mini-whip, and get a kiwiSDR (300 eur). Figure out how to pipe the IQ samples from the kiwiSDR over e.g. zeroMQ into your dev-machine that runs GNU-radio.This way you are decoding DCF77, MSF, TDF etc. in no time (fingers crossed...)this paper has a mini-whip-like preamp and uses the sound-card input (192 kS/s) instead of a 'real' SDRhttp://jmfriedt.free.fr/agu_dcf77.pdfsee also fig24 here:http://caxapa.ru/thumbs/417284/Engeler_DCF77.pdfthis one has an op-amp preamp:https://www.radioeng.cz/fulltexts/2013/13_04_1211_1217.pdf

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 19, 2020, 08:11:01 am
content: Quote from: awallin on February 19, 2020, 07:07:29 amWith what sort of latency, and how much jitter on the latency?The whole point of this sort of project is to get accurate time. There's very little point in compromising that by introducing multiple layers of post-industrial electronics... back in the day one of my lecturers quipped that the majority of undergrad projects were "...blah...op amp...blah...PLL..." but these days the craze for throwing microcontrollers into the mix makes things worse as far as just about every type of precision is concerned.MarkMLl

author: capt bullshot
author_rank: Super Contributor
timestamp: February 19, 2020, 08:22:24 am
content: Looking at your new schematic:The filter circuit looks OK now, and it requires the buffered virtual ground. C11 and C13 could be connected to GND instead of REF. You've put capacitors at the output of the LM358 buffers, this should either work fine or oscillate ...One could change the filter configuration to 3rd order Multiple Feedback and put the gain elsewhere, this would eliminate the need for the buffered virtual GND and should lower the GBW requirements on the filter OpAmps. Anyway, this is not nocessary to get the thing work, just an optimization to save an IC package (gain and filter in a 4 OpAmp package, or 2 duals).LM358 has a "quite poor" output stage (google LM358 crossover distortion). So if the load changes from pushing to pulling current (as it does when the virtual GND is AC loaded), it will generate noticable distortions as the output stage has to "move" the current from one transistor to the other output transistor. You'll notice that as spikes appearing on that virtual GND. Capacitors as you've connected in your circuit help buffering that, but in general OpAmps get instable if a capacitive load is directly connected to their output. A 10R ... 100R resistor in series usually cures this. If C17 is a ceramic, you won't need C18 at all, same for all the other 10uF / 100nF.Your circuit that outputs Vbias has the configuration of one divider loading another one. What's your intention with this circuit?Edit: mixed up Sallen-Key and Multiple Feedback filter topology. Your filter is Sallen-Key with gain, suggested to move to Multiple Feedback with "+" input tied to virtual GND.Edit 2: I'm a bit concerned about the way you've used the digital pot to control the gain. In theory, this should work for a quite wide range of gain, in practice, the parasitic capacitances and limited bandwidth of the pot might cause the OpAmp to oscillate. I'd suggest at least a smallish negative feedback cap from output to "-" here to cope with that.Another point: A single ceramic capacitor at the power input might cause excessive ringing when the power is plugged in. An electrolytic in parallel helps to save the live of U2.Next one: Now I see where you've put Vbias. I'd recommend to connect this node ("+") to your virtual GND as everything else, and place AC coupling between each amplifier / filter stage that provides more gain than one. In your configuration, small differences between Vbias and REF will quickly saturate the filter stages, as they are fully DC coupled. You'd need quite low offset amplifiers and Pin 5 of U4B at the very same potential as REF. AC coupling makes your life easier here.

author: faust1002
author_rank: Contributor
timestamp: February 19, 2020, 09:20:20 pm
content: @awallinI work on a physical layer of NR processing IQ samples in SW on a daily basis. I want to do something else, therefore I decided to design piece of hardware. Besides that spending over 300EUR to receive DCF77 signal seems like an overkill@MarkMLlEven the cheapest smartphone, when using LTE, is able to receive, decode and prepare a feedback for network in max 4ms. In NR this value can be even smaller, depending on a network configuration. Fair amount of processing is done by SW. DCF77 is by couple of orders of magnitude slower than LTE, therefore I wouldn't be worried about the latency introduced by SW.@capt bullshotFirst of all, thank you for some useful and detailed feedback. I really appreciate it.I updated my design based on your comments. Changes introduced in this revision includes:- C15 and C17 connected to GND instead of REF- AC coupling capacitors added between gain and filter stages- extra decoupling capacitors added for U1 and U2 (I don't like the idea of having electrolytic capacitors in my design)I really want to move forward and finally simulate my circuit. I believe that it is good enough (for sure not perfect). That is why- I didn't modify the filter topology- I didn't remove 10uF capacitors, which are in parallel with 100nF- Vbias remains as it was. I will probably never set DAC to value other than 0V- No capacitor was added between the output and inverting input of U4BProvided that I will not find any obvious problem in a simulation, I will design PCB (or two PCBs, one for the digital part, the other for analog) for my project and get it manufactured. Then, I will see what will oscillate and have so fun with debugging it!Besides that there is a maker's day on 13th March in my ARC. I would be great to have some hardware ready.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 19, 2020, 09:39:38 pm
content: Quote from: faust1002 on February 19, 2020, 09:20:20 pmStill got some remarks:R24, R27: place them between the LM358s output and the capacitors.Electrolytics: adding more decoupling capacitors won't resolve the issue I mentioned. I'm cautious in particular about C1 (the input cap), that's the place where you really should add some damping. Either use a 100uF (ballpark value) electrolytic in parallel to C1, or a series circuit of a ceramic and a 0.5R (ballpark) resistor in parallel to C1. Reason behind is, when you plug in the USB cable, the parasitic inductance of the cable in conjunction with the low loss ceramic input capacitor can cause the voltage ringing up to double the nominal value for a few 10 usec. Enough to kill your regulator U2, if it isn't rated for at least 15V input voltage (didn't look that up). Adding losses (by said electrolytic) de-fuses this.Your switches / pushbuttons are connected to short the supply. Doesn't look useful to me. Anyway, never connect a capacitor directly in parallel to a small pushbutton, that might kill the button over time.USBDN and USBDP appear to be shorted to VCCOtherwise, yes, fair enough, it's a good point to start. I wouldn't seperate the analog and digital part, separating breaks the recommended solid ground plane and must be handled with care.Just one more: your Vbias circuit won't output anything else but either "near GND" or "near VCC". This is due to missing negative feedback. My recommendation still is to connect U3A, Pin 3 and U4B, Pin 5 to REF and completely skip that Vbias circuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2020, 10:00:53 pm
content: I would not use a switched mode regulator with a radio receiver - especially if one does not have much experience. The safer way is a LDO. Such switched mode regulators are the typical sources of interference.The chosen OPs for the filters are really quite fast - this would need a really good layout and decoupling. If possible I would use a slower version - the same as for U3/U3 might be OK.The capacitors from the output of U6A and U6B to ground are causing trouble. They would need the 100 Ohms between the OP and the capacitor. There is no real need for C21 and C25.Even if one may not need those extra caps in the feedback at U3 / U4, it would be good to include them in the layout, so one could add them later if needed. The TS462 is dual OP, so U4 could be actually U3B.If at all there the capacitor in parallel to the switches should be smaller - chances are one can get away without them.For the supply to the µC and maybe the FT232 I would include some extra series resistors of some 10 Ohms from the general supply to the chip + decoupling.

author: faust1002
author_rank: Contributor
timestamp: February 19, 2020, 11:58:20 pm
content: Thanks guys for a prompt replies@capt bullshot- resistors R25 and R28 => corrected- electrolytic capacitors => ceramic C2 and C3 with electrolytic versions, the capacity of C2 increased to 100uF- USBDM and USBDP shorted to GND =>, corrected- switchbuttons => capacitors removed, resistors added- Vbias => U5A was supposed to act as a differential amplifier, but I must have flipped the op amp symbol upside down accidentally. Corrected@Kleinstein- Using LDO instead of a switched mode regulator => This version will be powered via USB and probably will sit about 10cm from my laptop. In other words, it will operate in noisy environment anymore. When I will switch to a battery powered version, I will consider your advice- Replacing ADA4891-2 with TS462 => ADA4891-2 was suggested by AD Filter Wizard. But I will simulate a version with TSP462 instead of ADA4891-2 and if results are good enough I will replace U3 and U4 with TS464. ADA4891 has much higher slew rate, but most likely I don't need this.- Additional resistors (R25 and R28) at the outputs of U5A and U5B added.- I knew that TS462 was a dual op amp, but I made a mistake in the symbol designator. Thank you for pointing this out.- The digital potentiometer I used has about 1MHz of bandwidth. Should be enough to not oscillate. If not I will have a lot of fun bodging extra caps in the negative feedback loops- What would be the purpose of the series resistor you mentioned?

author: capt bullshot
author_rank: Super Contributor
timestamp: February 20, 2020, 06:41:57 am
content: The differential amplifier will work as desired only if both inputs have the same source impedance. The "+" leg has 5k (the parallel of R19 and R20, google "Thevenin equivalent circuit"). The "-" leg is the MCU DAC - either something within 10k or rather low (buffered mode). So don't be surprised if this circuit doesn't do what you did expect in the first place.Read the datasheet of your SMPS IC to see what type and value of output capacitor is recommended and use this.The purpose of series resistors mentioned by Kleinstein is to achieve better decoupling of the noisy digital circuits from the supply voltage - say helping to keep the digital noise from propagating from the IC through the supply rail to other places. 10R would be a good starting point here, often works better than a ferrite bead.Someone mentioned that before, 56R resistors will make modern LEDs _very_ bright in this configuration. For a decent modern LED, calculate 2mA fwd current instead of 20mA.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2020, 07:22:18 am
content: The digital pots tend to have quite some capacitance at the "wiper" terminal. In the given circuit this would be at the inverting input and bad for stability. Just having 1 MHz BW for the digi pot is not enough to be sure a 12 MHz GBW OP will not oscillate. It is not for sure it will oscillate, but it is very well possible. If at all use a digi pot with few steps(e.g. 64) to reduce the capacitance and maybe consider lower resistance.USB Power is often noisy, but this is not an excuse for adding even more noise from an SMPS. With just a 5 to 3.3 V step there is not that much power savings, but more noise from the power supply. One will see if it really works close to the laptop - at least the antenna may very well need more distance (e.g. 1-3 m), especially with a receiver with limited filtering only in later stages.

author: faust1002
author_rank: Contributor
timestamp: February 20, 2020, 11:09:42 am
content: OK, I will add extra capacitors in the negative feedback loops of U3. Can you suggest a value I should use?I will also replace switched mode regulator with LDO.I will update my design in the evening.Edit: I updated my design. I replaced LM3670 with AMS1117. I also added C11 and C12 in the negative feedback loops of U3.I simulated the first two gain stages (using a fixed resistor divider instead of the digital potentiometer) and I am pretty happy with results I got.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 23, 2020, 09:20:17 am
content: So how many PCBs are you planning on ordering? :-)MarkMLl

author: awallin
author_rank: Frequent Contributor
timestamp: February 23, 2020, 01:26:19 pm
content: active antennas like mini-whip or magnetic-loop-preamp usually can be powered with DC over the antenna coax.if you plan on using that kind of antenna a bias-T for the DC-feed might be a good idea to add close to your antenna input connector?if your software processing figures out the received time wrt. to the ADC clock on the uC, would it be a good idea to clock the uC from an OCXO, and provide some means of tuning the OCXO (fine-resolution DAC?) - so that the OCXO and uC eventually phase/frequency locks onto DCF77?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2020, 02:07:00 pm
content: The filtering is a low pass filter only, just with rather large coupling caps the set a lower frequency limit. This setup would be quite sensitive to coupling from SMPS and similar at lower frequency. At least C10 should have to get a smaller values.The DAC signal send to the bias voltage does not make much sense. There should at least some way to disable it fully, so the µC DAC would not add noise to the input.The gain setting resistors at the filters are relatively low values. As noise is no longer so critical here one could use higher values to reduce coupling back via ground.The fast OPs also need local decoupling.

author: faust1002
author_rank: Contributor
timestamp: February 24, 2020, 10:11:02 pm
content: Hello guys,First of all, sorry for a late response. But I finally managed to simulate and design. I did a couple of changes- The first filtering stage => Sallen Key replaced with Multiple Feedback- The frequency response of the filtering stage changed slightly- TS462 is the only op amp used in the design@MarkMLlI am planning to order as little PCBs as possible. I hope that for about $50 I can get not only PCB manufactured, but also assembled and delivered.@awallinProbably I will use a piece of wire as an antenna. There is no need to complicate this project anymore. Furthermore, using OCXO would be another overkill in my opinion. I am going to use DAC to generate DC if needed (probably not). Moreover DCF77 signal is also phase modulated. That is why locking OCXO to it is still feasible, but not so straightforward. Finally, this is my pet project, not a commercial product.@KleinsteinI am planning to add extra digital filtering anyway, therefore I am not worried about out of band noise.Agree, I added this DAC signal just because I had it available. By default is will be set to 0VADA4891 are replaced with TS462. Unfortunately there are no layout guidelines for this component in its datasheet. But they are much slower than AD4891. Therefore I am not worries about local decoupling.Kind regards

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2020, 08:45:55 am
content: The TS462 also needs decoupling, even if there is no special board design guideline. Things are similar to other OPs.One can get away without decoupling with really slow OPs like LM358 and maybe 741.In addition to decoupling to ensure stability it is also about avoiding unintended feedback from the amplified signal. So even the 358 should get decoupling - I would avoid the 358 anyway, or give it some extra resistance to ground to make sure the output is not working in the cross over range.For the out of band signal it is not about about noise, but about a possibly strong signal driving the ADC/amplifier into saturation. The field from a SMPS can be be quite strong compared to the DCF77 signal. It depends on the local situation, but digital filtering may not help if the other signal is strong and only a 12 bit ADC. Even a good receiver may need a meter or so of distance to some SMPS.Just a wire as an antenna is not so easy at the low frequencies. The wires needs to be really long. So the usual antenna is a ferrite rod and a little less common a wire frame like used for AM radio.

author: faust1002
author_rank: Contributor
timestamp: February 25, 2020, 06:54:37 pm
content: Thanks for your reply. I will add decoupling capacitors for op amps.Yes, I am aware that strong interference caused by SMPS might be a problem. And the only good way of handling this issue is limiting your passband. But this is not what I necessarily want to do right now. Maybe in the future.About antenna, my main point was that I wanted to use something simple. As a ham radio operator I know how long a wire must be to use it as an antennaI agree with you that a rod antenna is a better choice. But I am not worried about this part right now.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: February 26, 2020, 11:08:29 am
content: Quote from: faust1002 on February 24, 2020, 10:11:02 pmAfter the number of (hopefully useful) suggestions from members of the community there might be others who'd like to be able to assemble your frontend circuit.MarkMLl

author: faust1002
author_rank: Contributor
timestamp: February 26, 2020, 09:03:11 pm
content: @MarkMLlI will release all design materials (Kicad project, LTspice files, SW, theory of operation, etc.) as soon as I will finish this project. This way anybody will be able to build or even modify what and how they like. This should be enough.I have just finished the new version. I believe I am good to go. Simulation results are as I expected, ECR passed. Now I will move to PCB design. Thank all for your help. I really appreciate.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2020, 09:17:22 pm
content: There likely is no need for the extra 4.7 µF cap for each OP. The larger capacitance (with some ESR - so just another 4.7 µF MLCC would not help) does not need to be close to each OP. Especially the Digi-pot and LM358 don't need it.For a first test it may still be OK to have space for a few extra caps, even of not initially populated. Just a footprint does not cost much.For the µC I would still consider an extra series resistor in the supply.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 27, 2020, 11:01:48 am
content: So good luck with your further progress, and I'm looking forward to your first results.