lna 77,5Khz , any advice - Page 1
Forum section: electronics / beginners

author: bst
author_rank: Newbie
timestamp: December 21, 2022, 06:47:47 pm
content: hi ,i want to build a receiver for dcf77 (77.5Khz).and I would like to have a frequency limited input stage.so i played a little in ltspice and found the following solution.https://www.eevblog.com/forum/beginners/lna-77-5khz-any-advice/?action=dlattach;attach=1669480looks too good to be realalso the deviations of the capacitors (+- 10%) and the resulting frequency shift (20 KHz)can be trimmed laterwhat do you thinkis the simulation ok?can it be built like this?any advice ?Thanks in advance

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2022, 07:52:47 pm
content: My opinion is, that all your JFETs and JFET opamps will add much more noise than you'll ever be able to filter out...

author: bst
author_rank: Newbie
timestamp: December 21, 2022, 08:37:56 pm
content: Quote from: Benta on December 21, 2022, 07:52:47 pmDo you think I should use other quieter types or should I do it without the jfets altogether?And is this noise something I can see in Ltspice in the basic transient simulation?mfg

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2022, 09:04:22 pm
content: No idea, never done noise analysis in ltspice.But what are you trying to achieve? Normally, 77.5 kHz filtering is done with the ferrite antenna, which you need anyway (unless you want to hang 20+ meters of cable out you window).

author: bst
author_rank: Newbie
timestamp: December 21, 2022, 09:43:47 pm
content: Quote from: Benta on December 21, 2022, 09:04:22 pmYes, I have a ferrite antenna for DCF77but the filtering of this is not sufficient, too many sources of interference are too strong.so i wanted to limit the bandpass as early as possible and not just amplify the antenna signal.and I hope that Ltspice maps the frequency behavior well enough with the given models and that I don't have any major errors in the simulation that prevent the circuit from working in real life.With regard to noise and Jfets, I have no experience and rely on what I determine in the simple AC or Transient analysis in Ltspice and the results are sufficient for my requirements if they are buildable

author: RoGeorge
author_rank: Super Contributor
timestamp: December 21, 2022, 09:51:10 pm
content: LTspice has noise analysis, but I don't think the noise would be a problem since you are closer from the DCF transmitter. I can receive it from Bucharest/Romania, with either a DCF clock or a DCF module I have, when the ferrite antenna is sited near the window.You should have an LC tuned antenna at the input, otherwise the first stage might saturate with stray radio signals, so the following filter won't help much. Both the DCF clock/weather station and the DCF module I have are with a ferrite antenna, something like this:Image from Wikipediahttps://en.wikipedia.org/wiki/DCF77Ready made DCF receivers were very cheap on AlieExpress when I bought mine.If you already have an LC ferrite antenna but didn't figured it on the schematic, then it might be OK. If you want a fancy filter, then try an N-path filter (AKA PolyPhase).International Solid-State Circuits Conference (ISSCC) prof. Bram Nauta explaining the basics and some examples of N-path filters:

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2022, 10:01:12 pm
content: The J111 JFETs are not that bad whent it comes to noise, especially if used with that much current. With quite some gain in the initial stage the noise of the 2nd stage is no longer that relevant.The resistor R7 at the input is likely a bad idea (a major noise source with the usual relative high impedance of a resonant antenna) - usually no need for this as a ferrite antenna has a DC path to ground.For the JFET choice the J111 is rather high in threshold and as a side effect shows quite some output side conductance. So the gain will be somewhat limited. For amplifiaction the J113 version would likely be more suitable.R4 and R5 are way on the low side and rather little gain for that stage. R20 and C20 is also to low impedance and loading the OP-amps way to much.Not many OP-amp models include noise. For the JFETs the noise simulation usually works, though not that well for the 1/f noise part. The noise part is a separate simulation. The transisent simulation does not include noise.With the DCF 77 signal it is often a good idea to have the first amplifier stage just at the antenna and a bit separate from the rest. This reduces the chance for unwanted feedback (e.g. via supply current).The power may come through the same coax as the signal.

author: bst
author_rank: Newbie
timestamp: December 21, 2022, 10:09:10 pm
content: Quote from: RoGeorge on December 21, 2022, 09:51:10 pmalso have a similar module, small very powerful beastthink that the module works with quartz filter for the bandwidth limitationQuote from: RoGeorge on December 21, 2022, 09:51:10 pmthe reference to exactly these videos in this forum is my starting point for this receiver, I would like to try to apply the method later in the mixer stagemfg

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2022, 10:15:20 pm
content: The N path filter is related to the Tayloe mixer. It could be a good idea to use some kind of mixing / IF to avoid having too much gain and a large signal at the base frequency.

author: bst
author_rank: Newbie
timestamp: December 21, 2022, 10:31:49 pm
content: Quote from: Kleinstein on December 21, 2022, 10:01:12 pmwithout R7 I have a second pole at around 800KHzwhich disappears at 10Kwithout R7R7 100kR7 10k

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2022, 10:35:26 pm
content: The circuit is way too complex to do the job.A low-noise opamp set up as a MFB bandpass filter will do the job just as well. There's plenty of power from the DCF-77 transmitter, it works around the world. My alarm clock had fine reception in the hotel room in California as I was there.77.5 kHz is easy for modern opamps.

author: bst
author_rank: Newbie
timestamp: December 21, 2022, 10:41:56 pm
content: Quote from: Kleinstein on December 21, 2022, 10:15:20 pmthat's one of my goals after some preamp and bandpass filteringmfg

author: bst
author_rank: Newbie
timestamp: December 21, 2022, 11:16:15 pm
content: Quote from: Benta on December 21, 2022, 10:35:26 pmIf I hadn't expected that we will send so much energy to get out of Europe.

author: ledtester
author_rank: Super Contributor
timestamp: December 22, 2022, 06:22:09 am
content: I realize this is a little off-topic, but today I just learned that cell phones are able to generate WWVB, DCF and other "time wave" signals, for instance see these apps:https://play.google.com/store/apps/details?id=jp.houryo.dcf77emulator&gl=US&pli=1https://play.google.com/store/apps/details?id=com.jntc.clockwavesync&gl=USThinking about it and applying some harmonics theory it seems plausible enough. The surprising thing is that the oscillating currents in the cell phone's speaker are strong enough to be picked up by a WWVB/DCF clock.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2022, 10:36:40 am
content: I am a bit surprized to see DCF77 reception in the US. Usually it is more for western Europe and with a good receiver northern Africa.A larger antenna and low BW detection could really help for long distance - so not impossible, but not easy either.If one really wants, one can reduce the BW quite a lot with SW based methods, as the time changes in a predictable way.It does not need much power to generate the frequency in the near field. It only needs the rather large antenna to get good far field emissions.In the old times there was quite some interference from the 5th harmonic of the TV horizontal deflection. This needed quite some effort in the filtering or just enough distance from a TV.Today much is interference from SMPS. The position of the antenna can make quite some difference.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 22, 2022, 02:34:36 pm
content: Maybe it's some local transmitter that also sends DCF.Should be possible to know if it's a local DCF or the one from Germany by comparing the DCF time versus the GPS time, then calculate if the time delay matches with the propagation distance from Germany to USA.

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2022, 08:22:19 pm
content: Dunno why you people are so surprised. LW and VLW transmissions extend around the globe, there's no need for LOS. Just ask the ham radio guys and BBC World Service.50 kW from the DCF77 xmitter is not a lot, but sufficient. The ham radio people reach South Africa on SW with 10 W from what I hear.The official ~2000 km radius is for guaranteed reception with a super-simple receiver, nothing else.

author: janoc
author_rank: Super Contributor
timestamp: December 22, 2022, 08:52:56 pm
content: Quote from: Benta on December 22, 2022, 08:22:19 pmHum, comparing HF and LW/VLW propagation is literally comparing apples and oranges, Benta.On the other hand, a clock doesn't need reception of the signal 24/7, it is plenty if it gets the signal in the middle of night once a week when the conditions are good to synchronize the time. That's why it likely worked even in California (even though US has its own transmitter WWVB at 60kHz).

author: bst
author_rank: Newbie
timestamp: December 23, 2022, 04:36:32 pm
content: Quote from: Kleinstein on December 22, 2022, 10:36:40 amadc -> memory -> fftwould like to test it later too , too many construction sites at the moment.I think after calculating the fft, the selection of the interesting frequency region is easier than removing the uninteresting ones in the signal path.Quote from: Kleinstein on December 22, 2022, 10:36:40 ammy desk led lighting is enoughjust don't manage to switch the protocolQuote from: Kleinstein on December 22, 2022, 10:36:40 amyes, there is a lot of information coming in through the antennapeople better than me could probably recognize the tv manufacturer.and smps are really very successful in preventing the receiver from functioning.all in all , i will give the circuit a change and build it up soon .will see ........Thanks to allwish you a nice Christmas

author: Kleinstein
author_rank: Super Contributor
timestamp: December 23, 2022, 05:34:51 pm
content: For a very low BW etection using some kind of SW defined radio is a good idea. However no need for a full FFT, it is more like doing a hilbert tranformation (kind of IQ mixer in SW) as a starting point.A point not often used is having a common sync for the start of all seconds. So it is not about individually detecting the long / short signal drops, but first synchronizing the second clock over a longer time interval if needed. Testing if the pulses are more a short pulse or long pulse or a missing pulse at the defined times is than a second step looking at the amplitude developement over some 1.5 seconds each around the times when the bits are expected to come. Ideally one also uses the information which "bits" are good and which are more uncertain. The way of looking at the result also reduces the effective BW, it is not just the analog RF filtering.A strong interference can saturate amplifier stages and maybe an ADC and this way suppress reception - a good position for the antenna really helps and thus ideally the antenna seprate from the main receiver part. One may also consider a way for adjusting the gain as needed - to much amplitude could also be a problem.

author: Benta
author_rank: Super Contributor
timestamp: December 23, 2022, 10:46:50 pm
content: Just to give an example of what is possible with an op-amp BP filter (pulled from my archives):(op-amp model: LM6171A, not the best choice here...)