Fw: [softrock40] Dynamic range measurements of SR V6

Terry Fox wb4jfi at knology.net
Mon Oct 2 11:56:10 CDT 2006

From the Softrock group.

----- Original Message ----- 
From: Marco 
To: softrock40 at yahoogroups.com 
Sent: Monday, October 02, 2006 12:38 PM
Subject: [softrock40] Dynamic range measurements of SR V6

Hi all,

I performed some more tests on IMD and dynamic range of the SoftRock V6 (original and modified). Here is a summary of the results.
For all measurements, the reference bandwidth is 2.4kHz, the signal spacing is 15kHz and the frequency is around 7050kHz. 

Instuments used: two HP 8640B signal generators, HP 8494A and -B step attenuators, Olektron power combiner. Levels checked with an HP 8566B spectrum analyzer. Power bandwidths and S/N measured with a Fluke 8920A digital RMS voltmeter. All instruments have traceable calibration.

1) Unmodified SoftRock, Soundblaster USB 16bit audio interface (16bit, 48kHz, better antialiasing and response than the usual AC97 found in PCs)
MDS: -105dBm
blocking dynamic range: 96dB
IMD3 dynamic range: not measurable (blocking occurs by saturation of the audio interface)
IIP3: -

2) same as (1), soundcard Audiophile 192 (24bit, sampling at 96kHz)
MDS: -110dBm
Blocking dynamic range: 100dB
IMD3 dynamic range: 93dB
IIP3: +29dBm

3) same as (2), post-mixer amp voltage gain reduced from 500 times (5kOhm/10Ohm resistors on the opamp) to 90 times (900Ohm/10Ohm)
MDS: -110dBm
Blocking dynamic range: 110dB
IMD3 dynamic range: 96dB
IIP3: +34dBm

4) same as (2), post-mixer amplifier has been replaced by a couple of OP27G powered from +/-12V. Opamp gain is now 200 times (2kOhm/10Ohm)
MDS: -110dBm
Blocking dynamic range: 115dB
IMD3 dynamic range: 102dB
IIP3: +43dBm

My comments:

a) I don't see any saturation in the FST3253 at least up to +10dBm (not checked over that)
b) the IMD products seem to come mainly from the opamp and audio interface. Low power, low voltage opamps are the limit here
c) some front-end gain is advisable to raise the MDS and stabilize the impedance seen by the mixer, if the opamp and audio interface can handle the strong signals. Noise from the opamp dominates the noise floor, of course; measured noise figure here is approx. 30dB
d) IIP3 figures appear very good, but the real performance is limited by the poor image suppression and the 2nd order audio harmonic generation in the chain. With only two tones at the input the spectrum becomes a real forest, such that it's difficult to find a clean spot to see the IMD products. So, probably we have to talk about SFDR (spurious free dynamic range) or the system. Much harder than a simple IMD3 measurement.

Next step: to test different mixer topologies (switch followed by an active integrator, as proposed on the HPSDR group, and some other ideas).

73 - Marco IK1ODO

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