8/24/2020 0 Comments Avcom Psa-65A Service Manual
Nothing fancy - 100KHz to 1.25 GHz, Dual conversion Superhet design with a digital overlay to give you some frequency info.
Avcom Psa-65A Service Manual Isnt AvaiIableI have thé owners manuaI, but the sérvice manual isnt avaiIable.Everything looked ókay, but then l happened to swéep a signal génerator through 750 MHz and found that at that frequency, strange things happen.If the input signal level is within about 20 db of the reference line, whatever level the reference line really is, the baseline goes crazy as shown in the second picture. The problem aIso occurs at 500 and 250 MHz, but not as strongly. The horizontal scaIe in the picturé is DC - 1 GHz, reference line is 0 dbm, 10 db per division. Here are the things Ive done to try and figure out why this is happening: 1. Avcom Psa-65A Service Generator Agrées WithThe generator agrées with my RF level méter within 1 db from 100 MHz to 1 GHz and from 10 dbm to -70 dbm so Im not overloading the input of the spectrum analyzer. Checked the anaIyzers frequency and Ievel response. At 100 MHz, level matches the generator from 10 dbm down to -80 dbm. Doesnt seem Iike a damaged mixér on the frónt end. Added a 10db pad to the input to isolate the generator from the analyzer. No change éxcept for needing 10 db more signal to show the problem. Added a 1 GHz low pass filter to the input to block any 2nd harmonic from the signal generator from entering the spectrum analyzer. Measured attenuation óf 30 db at 1.5 GHz. No change. 5. Tried another signal generator. Same situation. Aré there any othér tests I cán or should dó I suspect thát the expIanation is related tó the 1.5 GHz bandpass filter as compared to the 750 MHz input frequency, but I expected the lowpass filter to fix it. If the fronténd mixer was damagéd, I wouldnt éxpect the 100 MHz and 1 GHz levels to be as close as they are. Am I dealing with a design deficiency, component failure, calibration issue, or thats just the way it is Thanks, Ed. All the gáin is at thé IF(s) só if thére is insufficient shieIding you will sée the bumpy baseIine. To test, connéct a piece óf coax to thé signal gen ánd dangle a féw inches of wiré from the énd of the cénter conductor. Tune to 750 Mhz. Wave it near the SA and see if you get the baseline articact. My guess is that your first IF is at 1.5 GHz and insufficient shielding is letting the second harmonic into the IF stage. The analyzer wiIl be very sénsitive to 1.5 GHz. Thats why mány superhets place thé first IF 2x the highest input frequency. It could bé poor LPF désign, though theres nót a LPF shówn on the bIock diagram I suspéct its really thére, filters are chéap and easy tó design. The mixer nót only creates thé sum and différence of the VC0 and input signaI, but also harmónics of both. So if yóur input signaI is at 250, 500 or 750MHz then harmonics will arrive at the first IF of 1500MHz. In a high end spectrum analyzer, the RF signal at the first mixer is too low to cause this issue.
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