Re: True Received Frequency


Stephen Melachrinos
 

Steve -

I'll try to address some of this, but I'm not sure I'll get all of it. So if I miss something, let me know.

1. First, the simple part. As you said, if your radio says 14.070, the waterfall shows a tone at 1000, and you are using USB, then the tone is at an actual frequency of 14.071. If you are on LSB, then the tone is at 14.069. But this assumes that both the radio and the computer are "perfect." (See #2)

2. In reality, neither your radio nor the computer will be perfect, and both contribute to frequency errors. 
2a. The radio may not be exactly on the frequency displayed. You could use a calibrated signal source, either external or something like WWV, to find that. Note that the error could vary across the bands.
2b. The computer (waterfall display) could also have an error. I think this is because the waterfall's "frequencies" are actually derived from the sampling rate of the A/D converter in your sound card, and an error in the sampling rate results in an error in the displayed frequencies. Once you have the radio calibrated, you could use something like the FMT tones as a source to adjust the software to account for sampling rate errors.

Does this help?

Steve
W3HF


-----Original Message-----
From: Steve R via Groups.Io <oldjavadrinker@...>
To: 070Club <070Club@groups.io>
Sent: Mon, Apr 9, 2018 8:59 pm
Subject: [070Club] True Received Frequency

So I noticed last week that another frequency measuring test was conducted by K5CM and I assume sponsored by ARRL. Not really thinking the process through I measured the frequencies for 40 and 20 Meters using DM780 from HRD. Afterwards I realized that I would have and did make an error. If I am using USB to look at a signal, then I am really inserting an audio / side band error. 

So sure enough, I was 23 Hz off on 40M and 57 Hz off on 20M. So I noticed on DM780 you can select the frequency for the waterfall or select Hz from the base frequency. If you select frequency DM780 allows you to enter an offset to adjust the frequency across the waterfall.  Here are my questions?

Example, My radio is set for 14.070. I see a trace on the waterfall at 1000 Hz. This should translate to 14.071. But if we are using USB is there an error being inserted to the true received frequency?

How is that offset or error measured?   I would assume that since I am selected 14.070 USB, I am not really on 14.070 am I?

Something to change the subject and put on thinking hats...any thoughts?

Steve
VA3FLF
2301

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