Re: LF: Re: BBB-4-like receiver ready for first tests

[Daniele Tincani <danieletincani@yahoo.com>]

Hello Jim,

 

and first of all thank you very much for your help.

 

The Agilent was set for an amplitude of 50mVpp but I don't know if this setting applies to noise generation as well (I think I should study the instrument manual better :-)).

However, on SpecLab configured with "factory" settings, my soundcard usually shows a noise level < -140dB, when no input is connected to it.

Also consider that when I created a short circuit on the antenna input of the rx (see Q2 in my e-mail), I got a response curve on SpecLab similar to that I had with the Agilent, but with a peak level about 30dB lower (about -107dB around f=2KHz on SpecLab).

I also tried with a sine wave at 1KHz with the minimum allowed amplitude on the testset (50mVpp as said above) but this resulted in a lot of high-amplitude harmonics (n x 1KHz) at the output of the receiver, so I thought that may be even the minimum output level from the Agilent was too high for the rx input dynamic range.

Best regards

Daniele

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From: James Moritz <james.moritz@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
Sent: Mon, November 8, 2010 3:22:59 PM
Subject: LF: Re: BBB-4-like receiver ready for first tests

Dear Daniele, LF Group,

You wrote:
...>The spectrum taken at the Agilent output appeared to be pretty flat at -114dB on
SpecLab from DC to about 22KHz

The info missing from your test is what noise output level the Agilent AWG is set to - I imagine this would be volts RMS over some noise bandwidth. The -114dB level you quote is very low, and possibly just due to the internal noise of the sound card itself (my PC shows a level of about -118dB). Does the noise floor fall when the sound card input is disconnected from the generator? If the generator level is very low, this would account for there being little change in output level with the generator disconnected. Try increasing the generator output level - does the output from the circuit increase by the same amount? If you want to check the circuit gain, it would probably be better to use a sine wave input signal.

...>The resulting signal at the output of the receiver looked in good accordance
with the response curve I had obtained from circuit simulation in LTSpice.

Yes, that looks reasonable - so the circuit is probably mostly working, although without positively measuring the gain, it is still possible there is some fault around the input. I would expect an overall voltage gain roughly around 60dB.

...>Q1: could such a response curve be suitable for reception of sub-9KHz
transmissions in the dreamers band or for chasing whistlers only?

The filter rolls the gain off rapidly below about 1kHz and above about 10kHz. So this would be OK for whistlers and 9kHz reception, but would attenuate VLF utilities at higher frequencies. This could be a problem if you wanted to use a VLF utility for frequency-locking purposes. Reducing L1 to 22mH increases the upper cut-off frequency to over 20kHz - it also increases the in-band gain slightly too. C3, R4 and D1, D2 also will reduce the gain at higher frequencies, so you may wish to reduce R4.

Cheers, Jim Moritz
73 de M0BMU