HFU HF Underground
Technical Topics => SDR - Software Defined Radio => Topic started by: ChrisSmolinski on September 13, 2019, 1736 UTC
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I recently acquired a Hack RF One SDR. This SDR is somewhat unique in that it transmits as well as receives. It was designed primarily for use in the VHF and UHF bands, but I was interested in how it performed on the HF bands, possibly for use as as a QRP transmitter in the ham bands. So I decided to take some measurements...
The Hack RF One has an adjustable VGA gain of 0 dB to 47 dB, and an RF amplifier.
Measurements taken with a 50 ohm dummy load and a Rigol SD1052E scope measuring the RMS voltage and calculating the power.
With the VGA set to the maximum of 47 dB and the RF Amp off, I got the following power output values:
475 kHz: 0.027 mW
1.9 MHz: 0.15 mW
3.8 MHz: 0.23 mW
7.1 MHz: 0.26 mW
14.1 MHz: 0.22 mW
28.2 MHz: 0.27 mW
With the VGA set to the maximum of 47 dB and the RF Amp on, I got the following power output values:
475 kHz: 0.048 mW
1.9 MHz: 2.7 mW
3.8 MHz: 9.7 mW (distorted waveform, backing down to 45 dB gave 7.8 mW)
7.1 MHz: 24 mW (distorted waveform, backing down to 45 dB gave 16.5 mW)
14.1 MHz: 26 mW
28.2 MHz: 22 mW
Next to see the effect of varying the VGA. Yes, it does vary the output power. No, not exactly as much as you would expect from theory.
At 7.1 MHz, with the RF Amp off, measuring the output into a 50 ohm load:
47 dB 111 mV 0.246 mW
37 dB 44 mV 0.039 mW
27 dB 28 mV 0.016 mW
With the RF Amp on:
45 dB 880 mV 15.5 mW
35 dB 308 mV 1.90 mW
25 dB 109 mV 0.24 mW
15 dB 42 mV 0.035 mW
As a side note, the build in oscillator is pretty... bad. Off frequency. I just took a measurement vs my GPS 10 MHz reference, off by 71.3 ppm. Ouch. I have a high stability oscillator on the way. You can drive it with a 10 MHz reference input, which could be an option down the road.
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Thanks for doing this. I have been curious about this rig.
Next to see the effect of varying the VGA. Yes, it does vary the output power. No, not exactly as much as you would expect from theory.
At 7.1 MHz, with the RF Amp off, measuring the output into a 50 ohm load:
47 dB 111 mV 0.246 mW
37 dB 44 mV 0.039 mW
27 dB 28 mV 0.016 mW
With the RF Amp on:
45 dB 880 mV 15.5 mW
35 dB 308 mV 1.90 mW
25 dB 109 mV 0.24 mW
15 dB 42 mV 0.035 mW
I think what you mean is "not exactly as much as you would expect from the specifications."
The specs are likely only typical values, not max/min values, so don't be surprised if things differ "a bit" . (Everyone has a different definition of "a bit". ;) )
Noticing that with the amp off, the step from 47 to 27 dB (20 dB) was measured as 11.9 dB and with the amp on, that step was 18 dB. With the amp on, the the full step you have here (45 to 15 dB = 30 dB) was measured as 26 dB.
One of the things that happens with VGAs is that crosstalk from input to output can confound step characterization. Here's a table of your steps with the amp on:
45 dB --> 9.1 dB step from 35 dB
35 dB --> 9.0 dB step from 25 dB
25 dB --> 8.4 dB step from 15 dB
The incremental steps above are actually fairly uniform (but not exactly 10 dB) so crosstalk may not be the issue.
However, it is interesting that the step was closer to expected with the amp on, rather than off. Is there a possibility that the 27 dB value with the amp off is wrong? Are you measuring the noise floor of your oscilloscope at that step setting?
Any interest in a harmonic output or a two-tone IP3 characterization of the TX output? 8) :)