[hpsdr] TAPR Hermes - Noise on Audio Out

[John Marvin]

Ron,

As Dave has already suggested, the problem may be in the audio output 
circuitry.  What happens if you set the AF Gain to 0 in PowerSDR? Does 
changing AF gain affect the noise level? With the AF Gain set to 0 
PowerSDR should be sending 0's to your headphone/speaker, regardless of 
what is going on in the receive path. This will help determine if the 
problem is somewhere in the receive/decoding path or it is in the audio 
output path.

There just isn't that much in the audio output path. Any LAN issues 
(causing loss/corruption of packets) should not result in periodic 
noise. The FPGA then takes those packets, unpacks them and sends the 
audio to a TLV320 D/A (and A/D for mic/line input) converter over an i2s 
connection. The FPGA provides both the clock and data for this. This is 
the first point where the data is synchronous and any issues could lead 
to periodic noise. The TLV320 drives the headphones pretty much 
directly. A seperate set of outputs on the TLV320 drive an audio amp 
chip which is connected to speaker output(s).

Do you still get the noise with the LAN cable disconnected and just 
powering on the board with just headphones connected? Are you using 
headphones or the speaker output? If you are still getting noise with 
this test, try the other (speaker output vs. headphone output). Note 
that the speaker output is mono (it's just one of the output channels, 
both channels can be accessed via pins on the board) and differential, 
i.e. neither pin should be connected to a ground.

Any RFI issues that are strong enough to cause issues with the antenna 
disconnected or the RX input terminated may show up in the wide 
bandscope. Try running cuSDR and looking at the wide bandscope. If you 
have the receive antenna input terminated you should see only a 
relatively small number of spurs, and they should all be fairly low 
level, i.e. nothing that should be contributing to your problem.

A 2.8 ms period corresponds to 360 Hz, which is a multiple of 60 Hz. 
According to your testing I think you've probably eliminated any 
possible AC power issues, but still I'm always suspicious when problems 
come in multiples of 60 (in North America).

John
AC0ZG

On 11/20/2012 10:11 PM, Ron Nelson wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi,
>
> I have my TAPR Hermes working using PowerSDR, however, there is loud
> noise on the audio output independent of the RX input. The noise
> consists of sharp spikes, 2 V p-p with a period of about 2.8 ms. I do
> hear stations with an antenna connected, but the noise level is
> comparable to audio from strong stations.
>
> I think I have ruled out external local sources:
>
> Tried DC power from a switching power supply, a regulated lab bench
> supply and a battery.
>
> Connected Ethernet to PC directly and via a Linksys router.
>
> Tested with and without antennas. Always used 50 Ohm dummy load on Tx out.
>
> Tried two locations and turned off all other PCs and devices nearby.
>
> Put in the Anan case and grounded it. None of the above made any change
> to the noise.
>
> Suggestions please?
>
> 73
> Ron NR5ON
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