EMRFD Message Archive 14624
Message Date From Subject 14624 2018-04-13 22:26:21 stever1k Progressive Receiver I am building one of these fabulous radios and am looking for a Gain Distribution diagram for it. In particular the gain of the IF board and the Audio board. Wonder if anyone has those. I am having some sensitivity issues and can only barely hear a -80dbm signal when input to the IF board. Trying to determine if this is adequate...The 80m mixer board does not have much gain either..
14632 2018-04-16 10:22:24 kb1gmx Re: Progressive Receiver If all you can hear is -80dbm you missing a lot of gain!I know of no data specifying the gains. However having built it many decades agothe IF gain is very high on the order of 80 to 100DB. If you can't hear less than -80DBMyou have an significant issue with your construction. At that time I could not measure theIF directly but it was more than needed so the AGC system had the range of gain controlto do the job. FYI my test back then is if the tip of a screw driver induced noise to thevolume control it was good, more moise at the IF input was good, received signals atthe antenna input great!If memory serves I used a different audio line up after the preamp to be able to drive a speaker.The design as give has an easy 56DB of gain.FYI the AGC line on the dual fet IF devices should be in the range of 6V down to zerowhere 6V is maximum gain.Is the BFO working and on frequency for the filter chosen? Same for the VFO while I'm at it.IF stage oscillation is layout dependent and without good care that much gain will oscillate.The 80M converter (1ST mixer) is a low gain design and the line up if filter (-4 to-6db loss)DBM (-7db loss), Post mixer amp (+20db), post amp attenuator -6db for a net gain of around3-5db max. This is by design and the text explains that. to allow for the best overloadperformance the stage gain before the IF filter is in the range of mo more than needed toestablish the noise figure for the band, in short not a lot.If anything there is far more gain than needed. I found the one I made very hot RX even at80m where no RF preamp is needed. I dialed back the IF gain a bit.The HyCas IF strip ( Wes W7ZOI) is a drop in replacement or better using jfet and bipolartransistor pairs to replace the scarce dual gate mosfets. It works well and solves thedual gate mosfet issue well. That IF is in the 120DB range.FYI the current generation of simple superhet SSB/CW radios typically have less than 80dbgain from antenna to audio out. They hear to at least -120dbm or better. So lack of gain wouldhave to be severe in the progressive RX to be that tepid and likely not the reason.It was implied the Progressive RX could be built and tested piecemeal from the productdetector and audio toward the antenna. To do that you need the 80m input filter the80M VFO and the product detector and audio wired together as a DC RX, it shouldhear 80M signals well (less than -110db signals). If that works the IF and its filteris not working.Allison 14633 2018-04-16 11:04:21 Tayloe, Dan (Noki... Re: Progressive Receiver
I found that ~ 80 dB of total gain for a good low noise receiver works well when using a sensitive set of ear plug style headphones. ~100 mV pk-pk is a comfortable audio level for such headphones. Thus it is nice to use diode clippers to limit audio output higher than 0.4 to 0.8v pk-pk to protect the ears from sudden loud audio impulses.
I vaguely recall that driving to a speaker level required roughly another 20 dB extra gain. This extra gain is not really needed if you simple use a nice set of amplified computer speakers. I am using old Creative Labs T10 speakers but I understand that the T20 version is the more recent incarnation.
- Dan, N7VE
14634 2018-04-16 15:12:37 kb1gmx Re: Progressive Receiver Dan,True. I've built enough of them and lots of gain does not guarantee sensitivity or much else. I figured it out real fast many (n>4) decades ago when I connected the product detector that was said to have a sensitivity of .5uv to a crystal filter and prefixed with a simple downconverter of low gain. It was a eureka moment as then I realized that a good receiver didn't need a ton of transistors and IF stages with gain enough to oscillate.All that came from trying to make a good rx using just a 3n141 as a mixer, a filter out of an old HX50, and looking at a DC RX of the day that used a dual gate fet (3n141) as a product detector with a few stages of audio. It lead to me making a bunch of radios back then to explore low operating power while still sensitiveand had real selectivity.Taking a moment to go back. Our erstwhile radio builder constructing the Progressive Receiver and only heara -80dbm signal in a design where the product detector and audio section alone if working will hear a-100 dbm signal easily. Hence the missing gain comment. If the IF was working he would hear the collective IF device noise! So he has a problem, its not working as expected.However the Progressive is a single conversion with HF if and from the late 70s early 80s school were the goal was to put the high selectivity before much or most of the gain. This was before upconversion with wide first IF. The result was a lot of good receivers and transceivers that didn't tune DC to daylight but were strict ham band radios or heavily band switched to cover SW bands. Tentec, Atlas, and others are notables.All of those radios had a AGC system and gain for the main IF in the 80 to 120DB range and usually that much agc range as well. The front ends were built with just enough gain and often selective preselectors to give a reasonable if not low noise figure. Agc is nice to have but its easier to make a radio with low gain and an agc system that can produce negative gain like pin attenuators.How much gain is needed... Rick Campbell had a number based on what could be heard in a set of good modern 32 ohm headphones, about -40dbm (in a 50 ohm system). Put 80db around that and your can hear to -120 or so. Also all of the 80 can be at audio or RF or mixed.So now we have the mixer, filter, product detector and high gain audio line up radios. The easy low power form is a sa612 mixer, filter, and sa612 with an over all gain right near 30Db llowing for RF input and filter losses. Add a LM386 (20+db) and maybe an audio preamp (20db) and you can hear it if its there. The alternate is a switching mixer, maybe a post mixer amp, filter, another switching mixer and audio gain.Once the idea that for CW and SSB a lot of gain at some RF was not required the world took notice we have the radios we now can easily build and have fun with.Allison