Hi John, As always, this list takes a more practical approach than a theoretical one :-) Thanks for your reply. My question was more of a "Hmm, I wonder if such a device exists" type of question than one strictly about how one should build a receiver. This is for shortwave. I'm surprised by your "no one modulates below 300Hz" comment. AM SW broadcast can have excellent audio quality, and I'd be very surprised if their low-end cutoff were really 300Hz (you are probably thinking of SSB for communications use). If I don't do anything (i.e., just connect the audio amp to the mixer output with a termination resistor), my calcs show that my overall noise figure will be something like 12 or 14dB. While that would be adequate most of the time, a good HF receiver will have a NF around 8dB or so. Going lower than that CAN be useful sometimes, too, especially toward the higher end of the HF spectrum or when there are quiet conditions toward the lower end and reception is mainly by ground wave. Sean On Jan 6, 2008 11:05 PM, John Coppens wrote: > On Sun, 6 Jan 2008 22:32:40 -0500 > "Sean Breheny" wrote: > > > I have a very low-level (microvolt) audio source with a 50 ohm > > impedance (it is a diode mixer in a direct conversion receiver). I > > also have an audio amplifier which has a gain of about 10000 in > > voltage, 5 uV input noise, flat audio response down to about 10Hz, and > > a differential audio input with a 10K impedance. > > If the receiver is for shortwave, you wouldn't want to worry too much > about low-end audio or any quality at all (no one modulates below, say > 300Hz). Also, you will probably have enough (atmospheric) noise not to > bother with audio impedance adapting. Just put a 50 Ohm resistor as > terminator and connect the audio amp. > > On the other hand, at VHF or UHF frequencies, as far as I know, no USB > (or maybe AM. if the LO is synced) is used. So I'm not sure what > modulation you'd want to receive from a DC receiver? > > John > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist