DD4WH
Well-known member
Hi!
that looks like you have some sophisticated plans, very interesting!
Si5351:
* maybe you could experiment with the drive strength of the Si5351 which is software controlled? https://www.silabs.com/community/timing/forum.topic.html/si5351_output_impede-hiqv
* as far as I remember, you can also software-control the output capacity!? --> EDIT: this is wrong, the load caps for the crystal can be changed!
* And you could also try a 50 Ohms series resistor on the output
* I cannot report problems, probably because on my PCBs the maximum distance from the Si5351 to the Johnson counter is a few millimetres
IF:
* you are describing a very sophisticated setup which is difficult for me to understand, maybe there is also a misunderstanding of my concept of using an IF in *the digital domain*, NOT in the analog signal
I will try to explain the production of the spectrum display like it is designed at the moment:
* the Tayloe detector delivers a baseband signal which is already very well bandpass filtered with high Q (this bandpass filtering is an inherent feature of the Tayloe detector), so I think it will not be necessary to add another analog IF filter here
* we sample I & Q separately with a sample rate of up to 234ksps
* the I & Q seperately have to fulfil Nyquist, thus each deliver half of the spectrum display --> 234kHz / 2 = 117kHz
* together, I & Q give us a spectrum display width of a full 234kHz !!!
* So, there is no involvement of the digital IF up to now for the spectrum display
* the maximum available spectrum display width is always equal to the sample rate (which can be chosen by the user)
* NOW, for the demodulation of the signal, we internally shift the signal by [sample rate / 4] by multiplying the signal with a NCO [in reality, this is done without multiplication, but the principle behind is an NCO]
* after that, internally the signal that is demodulated is at baseband and can be treated by all kind of (convolution) filtering etc.
So, to conclude, we do not need IF filters of any kind, I think. But maybe I have not grasped correctly your suggestions.
The PCM1840 looks very interesting!
From my limited understanding -however- I do not think the main limitation would be the SNR of the ADC (which is pretty good with 99dB!), but the main limitation is the digital noise produced by the Teensy and the TFT, which is transmitted by the SPI wires and all kinds of clocks and received by the analog frontend.
Yes, your suggestion to use the Si5351 in quadrature mode for frequencies of 4MHz upwards and together with a Johnson counter for receive frequs lower than that is excellent!
73 de Frank DD4WH
that looks like you have some sophisticated plans, very interesting!
Si5351:
* maybe you could experiment with the drive strength of the Si5351 which is software controlled? https://www.silabs.com/community/timing/forum.topic.html/si5351_output_impede-hiqv
* as far as I remember, you can also software-control the output capacity!? --> EDIT: this is wrong, the load caps for the crystal can be changed!
* And you could also try a 50 Ohms series resistor on the output
* I cannot report problems, probably because on my PCBs the maximum distance from the Si5351 to the Johnson counter is a few millimetres
IF:
* you are describing a very sophisticated setup which is difficult for me to understand, maybe there is also a misunderstanding of my concept of using an IF in *the digital domain*, NOT in the analog signal
I will try to explain the production of the spectrum display like it is designed at the moment:
* the Tayloe detector delivers a baseband signal which is already very well bandpass filtered with high Q (this bandpass filtering is an inherent feature of the Tayloe detector), so I think it will not be necessary to add another analog IF filter here
* we sample I & Q separately with a sample rate of up to 234ksps
* the I & Q seperately have to fulfil Nyquist, thus each deliver half of the spectrum display --> 234kHz / 2 = 117kHz
* together, I & Q give us a spectrum display width of a full 234kHz !!!
* So, there is no involvement of the digital IF up to now for the spectrum display
* the maximum available spectrum display width is always equal to the sample rate (which can be chosen by the user)
* NOW, for the demodulation of the signal, we internally shift the signal by [sample rate / 4] by multiplying the signal with a NCO [in reality, this is done without multiplication, but the principle behind is an NCO]
* after that, internally the signal that is demodulated is at baseband and can be treated by all kind of (convolution) filtering etc.
So, to conclude, we do not need IF filters of any kind, I think. But maybe I have not grasped correctly your suggestions.
The PCM1840 looks very interesting!
From my limited understanding -however- I do not think the main limitation would be the SNR of the ADC (which is pretty good with 99dB!), but the main limitation is the digital noise produced by the Teensy and the TFT, which is transmitted by the SPI wires and all kinds of clocks and received by the analog frontend.
Yes, your suggestion to use the Si5351 in quadrature mode for frequencies of 4MHz upwards and together with a Johnson counter for receive frequs lower than that is excellent!
73 de Frank DD4WH