Front end for a swept spectrum analyzer  - Page 1
Forum section: electronics / projects

author: nenea dani
author_rank: Regular Contributor
timestamp: May 01, 2023, 09:53:49 pm
content: Hello colleagues, I present to you my adapter which, mounted at the input of an SSA3021x-TG spectrum analyzer, lowers the theoretical minimum frequency to a few Hz. However, I encounter a problem and that's why I want to describe the work. I have attached the diagram and then a photo with this adapter for compliance. I want to visualize the behavior of an amplifier equipped with electronic tubes in the range 30Hz-70KHz or 100kHz. As an analyzer with swept that lowers the frequency to a few tens of Hz is hard to find or rather it is expensive, then I chose the solution of adding a mixer in front of a regular analyzer. To test the amplifier, I use sinusoidal tones of 30Hz, 1000Hz and 20kHz for the beginning. First, the signal taken from the output of the tested amplifier enters the LPF with Fc=136kHz, followed by a 3dB attenuator and then into the IF input of an ADE-1H+ mixer. The output from the mixer is on the RF port and then another 3dB attenuator. As a local oscillator I have an SI5351, an amplifier with two BS170 transistors in parallel, a red torus filter with Fc=1.36Mhz and finally a 3dB attenuator. The two FTJ filters have low attenuation in the passband and then it goes down nicely. Obviously, I had to use the IF port to enter the mixer to accept a low frequency. The tests showed that for the best linearity and maximum output signal, I must have LO at the input to the mixer +16dBm and 90-120mVpp at the input to the audio filter. I am attaching a diagram with a 30Hz "sine" signal obtained from the output of my amplifier. The spectral component is as follows: Marker no. 1 is LO, marker 2 is the 30Hz fundamental tone, marker no. 3 is harmonic 2 and so on. The ratio of harmonics/fundamental tone of 30Hz seems realistic, but I encounter two major problems. The first is that LO is rejected too little, but it doesn't bother me. The worst thing is that the fundamental tone and harmonics appear extremely weak, i.e. below 30dBm. How can I improve the level or what other solution is there? Schematic is in second my post .

author: nenea dani
author_rank: Regular Contributor
timestamp: May 01, 2023, 10:01:48 pm
content: Unfortunately, my scheme was not attached and that's why I put it here. I apologize for the inconvenience.

author: nenea dani
author_rank: Regular Contributor
timestamp: May 18, 2023, 10:48:42 pm
content: Hello again,I tried to build several up-mixers, including the SA612, but they all suffer from pronounced non-linearity. The best solution as a higher output level (-32dBm) seems to be an H-mixer where I replaced the transformer at the RF-in input with a low-frequency one, for audio lines type A262A7E. I used a pcb board manufactured by SV1AFN.com. On the 20Hz-100Hz range, I get a monotonous signal increase of 0.7dBm, (6.24mV-6.83mV/50Ohm) which I think is partly due to the transformer and then the sinusoidal signal generator used for testing. Anyway, the transformer specification is a maximum of 1mW /30Hz. On the 30Hz-30kHz range, the level increase is approximately 7-8dBm, which is unacceptable. There is an input level for the signal that leads to an increase of less than 7dB but it is not a solution. I am attaching a simplified diagram because I have two questions. Should this transformer be one that has an air gap or not? The one I installed, that is, A262A7E, definitely does not have it, or at least I think so. What is causing my signal increase?

author: moffy
author_rank: Super Contributor
timestamp: May 18, 2023, 11:27:07 pm
content: If you want to analyze the 0 to 100kHz range, you should do it directly with a suitable A/D converter etc. Your method will introduce all sorts of distortion into the output because of the non linearity of the mixer stage and quite possibly the amplifiers, which will make a nonsense of any results.

author: Benta
author_rank: Super Contributor
timestamp: May 18, 2023, 11:36:21 pm
content: I think even a PC sound card would do the job. Never tried it myself, though.

author: nenea dani
author_rank: Regular Contributor
timestamp: May 19, 2023, 10:09:01 pm
content: Hello colleagues, I am attaching a photos while sweeping between 10Hz and 32kHz. A signal drop is observed as the frequency increases. Here is a link to a live video.https://photos.app.goo.gl/7xTiJXjV4ShMfpVS7There are no artifacts and there is sufficient dynamics down to the noise level. I think my audio transformer is the one causing the signal to drop. Now I have the primary windings in parallel, but if I put them in series, the signal weakens and the drop in relation to frequency is much steeper. Up to 20Hz there is no confidence in the measurement with RWB =100Hz because they are close to LO=1Mhz and this has phase noise that extends up to 10-15Hz. This is clearly visible if I sweep between 10Hz and 100Hz with a resolution of 1Hz.I have to somehow compensate for the loss of the transformer that increases with frequency, but I don't know how. I use a Focusrite 4i4 capture board for this experiment, but I'm not really satisfied because the sine signal starts to decrease slightly starting with 24-25kHz. Probably, in order to reach 80-100kHz, I need a card with ADC at over 300Khz, which is very expensive.

author: moffy
author_rank: Super Contributor
timestamp: May 20, 2023, 12:06:26 am
content: Well I am impressed, you have about 70db of THD+SNR which is quite reasonable, but not what you could get at baseband which would be around 100db. It looks like you have about -3db per decade rolloff from 1kHz to 10Khz and at around 25kHz it starts to roll off faster due to possibly the output filter of the Focusrite 4i4 (I have the Focusrite Clarett 2Pre on my desk). The low frequency restriction shouldn't be much of an issue as that is outside normal audio except subwoofers, and the phase noise limitation of your spectrum analyzer. Do you get that same -3db drop from 100Hz to 1kHz?

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2023, 01:02:06 pm
content: Quote from: nenea dani on May 18, 2023, 10:48:42 pmUse a normal double balanced mixer but swap the LO and RF ports. The LO port can operate down to DC.

author: nenea dani
author_rank: Regular Contributor
timestamp: May 20, 2023, 10:11:50 pm
content: Quote from: moffy on May 20, 2023, 12:06:26 amI will test what happens between 100Hz and 1000Hz with the best possible resolution. In the range 20Hz-100Hz I have, as we said above, a difference of 0.7dBm which is surely due to the low frequency transformer for the audio line. For this reason, I asked if the lack of air space can cause the core to enter incipient saturation. On the 100Hz-1kHz range, I don't think I have 3dB attenuation, probably less, but I will check.Quote from: David Hess on May 20, 2023, 01:02:06 pmUse a normal double balanced mixer but swap the LO and RF ports. The LO port can operate down to DC. I will try on Monday to build another mixer in the indicated configuration.

author: moffy
author_rank: Super Contributor
timestamp: May 21, 2023, 02:57:45 am
content: Quote from: David Hess on May 20, 2023, 01:02:06 pmDavid Hess mentioning a doubly balanced mixer reminded me of an old project I built where I used an MC1496 as a precision VCA, managed to get -80db THD but not the -100db I was looking for. Anyway the MC1496 is a doubly balanced mixer and I used a couple of techniques to improve its THD. I added U1A & U1B to linearise the audio input, which worked quite well. I used Cin+ & Cin- as the voltage control, but you would use that for your modulator signal. You would need to upgrade U2 if you want to use opamps or you could just replace them with resistors going to the +ve supply. I've included the schematic in the hope that some part of it might prove useful.

author: nenea dani
author_rank: Regular Contributor
timestamp: May 21, 2023, 06:31:23 pm
content: I think I will use part of the proposed scheme. How did you find out that the 1496 is not linear enough that you entered the NFB?

author: duak
author_rank: Super Contributor
timestamp: May 21, 2023, 07:48:07 pm
content: Bună ziua!You may wish to to consider the Analog Devices AD734 4 quadrant multiplier:https://www.analog.com/media/en/technical-documentation/data-sheets/ad734.pdfThey are expensive, but have 10 MHz bandwidth and are laser trimmed. I have used them in an instrument for real time power measurements of varying frequencies, amplitudes and waveforms.Cheers

author: nenea dani
author_rank: Regular Contributor
timestamp: May 21, 2023, 09:17:26 pm
content: Quote from: duak on May 21, 2023, 07:48:07 pmSalut, Hello,Actually, when I started this project, I also saw AD834, which I could easily buy compared to AD734 from Farnell, but I was afraid that it is a bit exotic for me. I also have the problem that rare integrated circuits need 2-3 days to arrive. It seems that this will also be an option if the H mixer ends badly. Now I'm waiting for two transformers, one with an air gap and the other without, with a specification of 15Hz-80KHz. These are made to order in Poland and still take a few days. What puzzles me is that every mixer I tried, whether it was double balanced or Gilbert, that is, with loss or gain, they greatly attenuate the mixed signal. Less technically speaking, I introduce dozens of millivolts or 100mV /1000Hz into the mixer and at the output I get a maximum of 0.5-1mV in 50Ohm. I don't have a safe solution for an audio signal analyzer. Capture cards are very expensive and I'm afraid they are not far from Focusrite, second hand 3-10Hz analyzers are full of bugs as I experienced with one (10Mhz-12,6Ghz) and something new costs as much as two cars. Picoscope 4262 remains the last solution, as someone said above.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2023, 09:39:23 pm
content: The QA403 audio analyzer specifications say that it has a low frequency cutoff below 1 Hz:https://quantasylum.com/collections/frontpage/products/qa403-audio-analyzer

author: moffy
author_rank: Super Contributor
timestamp: May 21, 2023, 10:43:31 pm
content: Quote from: nenea dani on May 21, 2023, 06:31:23 pmIf you look at the equivalent circuit for the MC1496, it is just a Gilbert cell, doubly balanced, and I applied negative feedback via U1a and U1b around the BE junctions of the transistors driving R9. That removes the nonlinearity of the BE junction with current and improves the THD dramatically.

author: nenea dani
author_rank: Regular Contributor
timestamp: May 22, 2023, 09:57:37 pm
content: Quote from: David Hess on May 21, 2023, 09:39:23 pmI read the documentation but I did not understand if it can make measurements above 20kHz. I want to test at least up to 50kHz, but I aim to reach 70kHz so that I can see the 5th harmonic for the 14kHz sounds. Numerous tests done by renowned laboratories show that 14kHz is the upper limit up to which the general public can hear. It is a statistical test and has nothing to do with the maximum limits of human hearing.

author: moffy
author_rank: Super Contributor
timestamp: May 23, 2023, 02:04:11 am
content: Quote from: nenea dani on May 22, 2023, 09:57:37 pmI looked at the data sheets of the qa403 and the loopback tests have about 22kHz BW even at 192ksps.

author: nenea dani
author_rank: Regular Contributor
timestamp: June 28, 2023, 12:27:25 pm
content: Hello colleagues, I built a new mixer that uses the AD835 multiplier. I am attaching the schematic and some diagrams to 100KHz and then a detail with 0-110Hz. I have an error of 4Hz, that is, instead of 1MHz, I actually have 996kHz from the local oscillator. I used a white noise generator fromhttps://www.edn.com/white-noise-source-flat-from-1hz-to-100khz/As you can see, I have a constant level on the entire audio band, even up to 100kHz. However, I do not have enough output level, more precisely there is no dynamic room between the situation with the noise generator connected and when it is disconnected (-76dBm vs -125dBm). I would need at least 70dB. Even if the signal from the white noise generator reaches 400mVpp, I still don't get a higher dynamic. I also inserted a resistance of 2kOhm between pins 4 and 5 but without result. It is curious that the local oscillator appears very strong at the output as you can see. Something is wrong and I don't know what is happening.

author: moffy
author_rank: Super Contributor
timestamp: June 28, 2023, 10:23:08 pm
content: Perhaps a better test would be to apply a sine wave instead of the white noise and see what your dynamic range is then. From what I understand you are trying to measure harmonics. With white noise the peak to peak excursions limit the amount of power you can put in, 400mv p-p is about 40mv RMS, that disparity might be the reason for your apparent loss of dynamic range.

author: gf
author_rank: Super Contributor
timestamp: June 29, 2023, 08:56:12 am
content: White noise has a spead spectrum and the total noise power is not concentrated in a single peak, but spread evenly over the whole occupied bandwidth of the nose signal. For instance, if we assume white noise with a bandwidth of say 0...100kHz and a total power of -15 dBm, then the corresponding power spectral density is only -65 dBm/Hz. Don't know the ENBW of your SA's filter, but for simplicity let's assume 2 Hz ENBW at 1 Hz RBW. Then the SA "sees" only 1/50000 of the total noise power at each measured frequency point (i.e. -62dBm), because only this fraction of the total noise power falls into the filter's passband. OTOH, if the signal were a -15 dBm sine wave, the SA would see a single peak with -15 dBm.

author: nenea dani
author_rank: Regular Contributor
timestamp: June 30, 2023, 11:02:17 pm
content: The main reason why I use the white noise generator is to find out the bandwidth of an audio amplifier with electronic tubes. For the study of harmonics, obviously I will use a sine generator from Focusrite 4i4 which does not "climb" well above 30kHz. We don't think it reaches 100kHz.moffy & gf you are absolutely right that the noise is distributed over the entire band. I am not familiar with the whole mathematical apparatus, but I know the principle because I use a generator called Xgen-v1 between 5-12GHz fromhttps://g8fek.com/noise-gen.htmlI have a super old Systron Donner 809-1 that I am constantly remanufacturing.I got stuck because now it's about the audio frequency and I forgot that all the energy of the generator is distributed. Thank you both for guiding me in the right direction. With only 250mVpp sine signal now I get a dynamic of 80dB. Now I have to do the first tests with a tube amplifier.

author: moffy
author_rank: Super Contributor
timestamp: June 30, 2023, 11:19:01 pm
content: You are welcome, I am enjoying following your progress.

author: nenea dani
author_rank: Regular Contributor
timestamp: July 02, 2023, 03:23:48 pm
content: Hello colleagues, I am attaching two diagrams with the mixer that receives a 1kHz sine signal through an uncompensated resistive divider in frequency from the output of an SE amplifier with EL34 in the triode for 2.5W/8Ohm. The next two diagrams are for the signal from the white noise generator for an output level at an unknown level, but probably still around 2-3W. For this last case, how do I estimate where the upper frequency of the -3dB corner is? I know that I should have shown the frequency on a narrower band, but I am interested in the procedure or method of calculation. I suspect that it is somewhere between 20-30kHz.

author: moffy
author_rank: Super Contributor
timestamp: July 03, 2023, 06:31:07 am
content: It is a bit difficult to tell from the noise plot as it appears to have an almost constant slope, and it randomness makes it difficult to judge accurately. A swept sine, or just plotting the output vs a constant level sine input at different frequencies should give better results. Start at 1kHz, then every octave, when you suspect you have found the 3db point you can then sweep with a bit more resolution.P.S. Looking at the thumbnail it looks like there might be a change in slope between 40kHz-60kHz.

author: nenea dani
author_rank: Regular Contributor
timestamp: July 03, 2023, 11:08:13 am
content: I chose the solution with the white noise generator because I have nothing else reliable for more than 20-25kHz. If I use the averaging function after many successive passes, I will get a fairly straight line. Can I proceed in the same way as in the case of radio frequency for fc=-3dB where the unit of measure is dBm? My problem is that this unit (dBm) is not used or somehow not accepted in the audio. I see everywhere diagrams from audio amplifiers with some level in dB vs. frequency, but from which units of measurement come these differences that give these -3dB?

author: moffy
author_rank: Super Contributor
timestamp: July 03, 2023, 01:00:49 pm
content: The value -3dbm is an absolute level with respect to 1mw of power. The -3db point or bandwidth of a filter is a relative measurement of a drop in gain, so is relative and not absolute, and with regards to a low pass filter is normally followed by a slope of some multiple of -20db/decade of frequency.

author: nenea dani
author_rank: Regular Contributor
timestamp: July 15, 2023, 11:33:08 pm
content: Hello colleagues, I built a down converter that receives a signal from the spectrum analyzer generator (TG) between 1MHz and 1,100MHz. The local oscillator is 1MHz and therefore I get a very clean sine signal at the output regarding the harmonics. I am attaching the principle diagram with AD835 which receives 820mVpp from LO and 650mV from TG. At the output of the LPF filter I get only 30.5mV and here is one of the problems that must be investigated. I am also attaching the diagrams provided for 23Hz, 1KHz and 100kHz taken with the scope input set to 50Ohm. The output voltage is around 30mVpp with a random variation of approx. 0.5mV due to various causes. These are the noise of the TG signal, various RF disturbances from the devices in the laboratory and the LED lighting that enter the connections and the unshielded coils of the LPF. The most important source of variation is the 50Hz pulsation and this is what I noticed for the settings with the longest time base. I use SSA3021X which in TG mode does not allow RBW below 30Hz, so below 50-60Hz nothing is to be taken into consideration. The user manual refers to TG starting with 100Khz upwards and the variation can be +/-3dB. Indeed, although TG starts with a few kHz, the signal is distorted and very small below 60-65kHz. I have a doubt about the resistance marked Rx at the output of AD835. The AD835 circuit needs an optimal resistance of 150 Ohm, but the LPF must see 50 Ohm at the input. It will probably have to be 100 Ohms and thus the level of the output signal will increase. The SA has a 10MHz BNC output at least 0dBm and I think I should use a 1/10 divider and a signal shaping amplifier or something to provide 1MHz for both down and up mixers. As I work, I begin to understand why these didactic SAs do not go down in the low frequency range or at least not in TG mode.

author: nenea dani
author_rank: Regular Contributor
timestamp: July 15, 2023, 11:36:58 pm
content: I am attaching the diagrams for 23Hz and 1kHz below.