How to correct the frequency offset of the TCXO (RX888) ?

[John DF4UE]

I own two RX888MK2 and the 27MHz-TCXO of both of them has a slight offset. Appr. 5-7 Hz.
Therefore the setting of the sampling frequency i.e 60000000Hz will also show an offset.

Which setting in PhantomSDR is responsible for the clock frequency generated by the Si5351A?
Is it in the "config-rx888mk2.toml" or in the  "./start-rx888mk2.sh" ?

Regards
John

[Bas ON5HB]

I looked in the driver, in VHF mode you can give a tuner frequency, but I can't find any parameter or setting to do it for HF.

Code Select Expand

VHF {
        /// Tuner Frequency
        #[arg(long, display_order = 100, default_value_t = 145000000)]
        frequency: u64,


So I believe you must/can give a frequency after the VFH parameter, maybe also after the HF parameter.
Don't know. You have to try.

Else set the offset in the Toml file by giving a slight off-value.
You have to try, maybe a base freq of -/+ 5 to correct 5Hz?

[John DF4UE]

Hi Bas,
tnx for your answer.
I did a small change in the start-rx888mk2.sh without doing this change in the toml file.
Doing this I can correct the offset i.e. at 10 MHz, but at 30 MHz there is still an offset.
If I do it for 30 MHz, I get an offset at 10MHz.

There is no setting possible to do the correction for the whole spectrum. Strange.
However, just for SSB or most other modes the small offset is irrelevant.

John

Btw: I use a Bodnar GPSDO with 5 MHz clock and can see the harmonics with good signal every 5MHz up to 30MHz.

[Bas ON5HB]

Have a look here, Maasbree is using this too, but it seems you can't connect the Bodnat to the RX888 without few extra parts.

https://websdr-maasbree.jouwweb.nl/websdr-maasbree-2-0/control-the-rx888-mkii-with-a-bodnar-reference-clock

Also, you should use a clock of 27MHz.

Maasbree has it spot on the Hertz as you can get.

[John DF4UE]

This was the next thing I wanted to do.
Now, with 27.000000 Hz external clock from the GPS locked Bodnar the offset at 27 Mhz is still + 1 Hz.
So, this seems to be caused by something else....

Any ideas very welcome.

regards
John

[Bas ON5HB]

Did you see this on their page?

We recommend a newer version of this product: LBE-1420 GPS locked clock source

This device outputs low jitter reference clock frequency-locked to GPS signal.

Long term stability of output signal is defined by high accuracy of GPS Caesium references and approaches 1x10-12.

Short term signal quality is defined by internal TCXO clock source providing high-quality, low phase noise clock signal with sub-picosecond RMS jitter.

https://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301

[John DF4UE]

I know this, but I think it has no relation to my issue with the offset.

Btw. the leakage of the own TCXO of the RX888 can be received very weak on 27 MHz. And it has also an offset, which is not logical to me (see attached file).

But this is no big problem, I was just curious if somebody knows the reason behind.

regards
John

[John DF4UE]

I just checked other PhantomSDRs with RX888 on the Web and they show the same effect.

[Bas ON5HB]

Mine doesn't....no signal at 27Mhz.

[Bas ON5HB]

I do see a lot of unwanted signals:

You cannot view this attachment.

Maybe common-mode-currents?

[John DF4UE]

I checked again with spectrum display activated.
The peak of the signal in the spectrum display is exactly at 27.000000MHz. Just the trace in the waterfall is shifted.