Thank you Dan and Jim for the good comments. Sure I noticed how difficult it
is to measure the sound cards without proper instruments. The clipping (or
compression) levels are easy, but the noise in the present computer
environment and with signals approaching the thermal noise levels are
challenging.
Instead of measuring the audio card only I decided to continue with the
whole SDR-1000 system. I recorded 1) the noise floor (dBm/500 Hz) with audio
card input cable input connected to the radio and the antenna connector
terminated to 50 ohm and then 2) with a signal to radio until clipping or
compression was indicated at the line-in connector of the radio or at the
SDR-1000 own measurement systems.
The results were:
Preamp Setting HIGH, -140 dBm/500 Hz, -26 dBm, INA163 out 25 Vpp
Preamp Setting MED, -130 dBm/500 Hz, -16 dBm, INA163 out 25 Vpp
Preamp Setting LOW, -130 dBm/500 Hz, -13 dBm, INA168 out 4.8 Vpp (1.4 dB
compressed)
My conclusion is that the QSD can take about 4 Vpp until it starts to
saturate and my sound card can take 29 Vpp, so the amplifier after the QSD
could have 17 dB voltage gain for optimal results. The front end gain need
to be adjusted accordingly. Dan mentioned:"... ideally 130 to 145
db to match the blocking performance of other rigs...". This should be our
target and to achieve that we need audio cards handle signal from tens of
nanovolts to tens of volts.
I estimate, the accuracy of the above measurements is about 1 dB. The
measurements were made with PowerSDR 1.4.5 console with unmodified RFE.
These figures serve as the reference when comparing the results of the
ECO-25 modifications.
73, Ahti OH2RZ
----- Original Message -----
From: "Jim Lux" <[EMAIL PROTECTED]>
To: "Tayloe Dan-P26412" <[EMAIL PROTECTED]>; "Tayloe Dan-P26412"
<[EMAIL PROTECTED]>; "Ahti Aintila" <[EMAIL PROTECTED]>;
<FlexRadio@flex-radio.biz>
Sent: Saturday, September 24, 2005 6:20 AM
Subject: Re: [Flexradio] Audiocards, USB, etc.
At 05:40 PM 9/23/2005, Tayloe Dan-P26412 wrote:
I would think that on top of the audio blocking test we also want to
run audio IP3 tests and audio IP2 dynamic range tests as well to make
sure that the distortion characteristics are at least as good as the
SDR1000 front end.
- Dan, N7VE
Such tests would be useful, but are quite challenging to make for high
performance systems. You can take two general approaches:
1) Obssessively account for all the error sources, use very clean sources,
etc. so you can truly know that what you've measured is just the unit
under test, and not quirks of the experimental setup.
2) Measure it in a "typical setup", in which case the performance will
certainly measure out worse, but at least it's representative of what
you'll really get.
Consider that if you're looking for 140 dB relative levels, you're looking
for signals of a 0.1 microvolt on a 1 volt signal. I've done DC
measurements to 6 digits, and it's, frankly, an ordeal.
You'd also need sources that are that good, which is no easy matter,
especially if you want to cover the full frequency span of the device
(several decades). For instance, the SRS DS360 claims <-100dBc distortion
from 10mHz to 20 kHz. It's not too pricey at about $3000.
