Last night I decided to run a few tests. I don’t have an elaborate lab, but I do have two calibrated signal generators and some other test equipment.
What I had read about the SoftRock not responding to signals on even harmonics of the LO frequency is not true according to my tests. First I looked at MDS (minimum discernible signal) at 474 kHz (LO frequency + 10 kHz) with a signal generator connected to the receiver. I could still clearly see the signal in HDSDR at -135 dBm. Next I measured at 938 kHz (2 x LO frequency + 10 kHz) and got -57 dBm. At 1402 kHz (3 x LO + 10 kHz) I found it to be -53 dBm. There are some things I don’t understand about this result. Theoretically at 1402 kHz it should be down no more than the SoftRock front end filter attenuation at that frequency plus a few dB because the LO third harmonic is 10 dB down from the fundamental. According to my modeling of the filter and spectrum analyzer sweep of the same filter outside the receiver, it attenuates 41 dB at this frequency. 41 plus 10 is only 51 dB down according to theory. Yet I was seeing 82 dB difference in MDS. That doesn’t make sense to me. What is clear both from this test and from those broadcast stations I was able to identify while listening on the SoftRock is that both the second and third harmonic response is considerable.
I spent several hours carefully examining the strength and number of broadcast signals making it through the front end selectivity. My external bandpass filter removed all traces of broadcast stations being heard with the receiver. Its calculated and measured response is -60 dB at 900 kHz, -84 dB at 1400 kHz. The filter I plan to build for the SoftRock should be -66 and -88 dB respectively.
I attempted to run a close spaced IMD test on the receiver. With signals on 473 and 474 kHz, I detected no IMD products when both of these signals were 90 dB above the noise floor. This is probably sound card dependent but it indicates to me that the SoftRock hardware has good strong signal performance. I reduced one signal to barely audible and left the other at 90 dB above noise floor. Switching the strong signal on and off did not noticeably affect my ability to hear the weak signal. I need a different test setup to go beyond 90 dB in these tests, though I am satisfied it is as good as it needs to be.
This continues to look like it should be a relatively low cost, high performance 630 meter receiver once the added front end filtering is in place.