Theory & Poor Receive. What should I expect around U6 / U9?
Posted: 25 Apr 2022, 22:53
I’m not hearing any CW or USB / LSB activity on any bands so I’m trying to debug my boards. I do hear static; the noise floor. I also hear occasional strong tones as I tune around, but I think these are just hash from local electronics.
Antenna is a magloop. My FT817nd hears reasonable numbers of people transmitting, CW and USB / LSB, with the magloop appropriately tuned.
I’m suspecting problems in the U6 multiplexer or U9 op amps area. I will have an oscilloscope soon to look at what’s going on.
My plan is to inject a fairly weak signal, sine wave at perhaps -80 dBm, and snoop through the signal path. This should simulate a continuous CW signal, with no additional modulation. I can do this with the tinySA spectrum analyzer (see https://www.youtube.com/watch?v=RvYwAn32Dl4 starting at about 2 minutes).
My understanding of the schematic is this: SMA connector feeds the 1-turn primary of T1. Then signal goes through the selected band’s relay and bandpass filter. From there it goes through the pin marked ‘RF’ of the J3 inter-pcb connector to the main board. On the main board, signal passes to Q1, the Tx/Rx switch. During receive, signal goes to pin 9 of the U6 SN74CBT3253 multiplexer/demultiplexer. In there, the Tayloe detector magic happens.
Here’s a YouTube video with what looks like a very similar Tayloe detector circuit:
https://www.youtube.com/watch?v=iwBk3jIxszI
If I understand correctly, the output of the U6 multiplexer plus the capacitors, should be the audio signal expressed as a quadrature (two waveforms 90 degrees out of phase).
For a CW signal there isn’t any audio modulation. What should I expect to see on the outputs of the demultiplexer? What should I expect if I look at the outputs of the U9 op amps on the ‘scope?
Then I intend to do it all again, but injecting a 1kHz amplitude-modulated signal on the same carrier frequency, again using the tinySA device. With a 1kHz AM signal, what should I expect on the outputs of the U6 mux/demux, and at the outputs of the U9 op amps?
If I sound like I know what I’m doing, well, I’m faking it. I’ve never owned an oscilloscope, though I know approximately how to use one. And the tinySA is new to me as well.
Thanks everybody! This is a fun project.
Antenna is a magloop. My FT817nd hears reasonable numbers of people transmitting, CW and USB / LSB, with the magloop appropriately tuned.
I’m suspecting problems in the U6 multiplexer or U9 op amps area. I will have an oscilloscope soon to look at what’s going on.
My plan is to inject a fairly weak signal, sine wave at perhaps -80 dBm, and snoop through the signal path. This should simulate a continuous CW signal, with no additional modulation. I can do this with the tinySA spectrum analyzer (see https://www.youtube.com/watch?v=RvYwAn32Dl4 starting at about 2 minutes).
My understanding of the schematic is this: SMA connector feeds the 1-turn primary of T1. Then signal goes through the selected band’s relay and bandpass filter. From there it goes through the pin marked ‘RF’ of the J3 inter-pcb connector to the main board. On the main board, signal passes to Q1, the Tx/Rx switch. During receive, signal goes to pin 9 of the U6 SN74CBT3253 multiplexer/demultiplexer. In there, the Tayloe detector magic happens.
Here’s a YouTube video with what looks like a very similar Tayloe detector circuit:
https://www.youtube.com/watch?v=iwBk3jIxszI
If I understand correctly, the output of the U6 multiplexer plus the capacitors, should be the audio signal expressed as a quadrature (two waveforms 90 degrees out of phase).
For a CW signal there isn’t any audio modulation. What should I expect to see on the outputs of the demultiplexer? What should I expect if I look at the outputs of the U9 op amps on the ‘scope?
Then I intend to do it all again, but injecting a 1kHz amplitude-modulated signal on the same carrier frequency, again using the tinySA device. With a 1kHz AM signal, what should I expect on the outputs of the U6 mux/demux, and at the outputs of the U9 op amps?
If I sound like I know what I’m doing, well, I’m faking it. I’ve never owned an oscilloscope, though I know approximately how to use one. And the tinySA is new to me as well.
Thanks everybody! This is a fun project.