UHF Frequency problem.

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g1hmo
Posts: 4
Joined: Thu Aug 11, 2022 3:18 am
Location: Staffordshire, UK

UHF Frequency problem.

Post by g1hmo » Thu Aug 11, 2022 3:50 am

Apologies for the long email, but I wanted to capure all relevant information in the hope that it may help, I am having an issue with the UHF frequency on my MD-9600 I recently flashed withOpenGD77 frmware.
I have had the radio for a couple of years running the standard TYT firmware and have had no issue using it in DMR with my MMDVM hotspot.
I found out recently about the GD77 firmware, and was keen to give it a go, firstly congratulations on a superb improvement over the TYT firmware, it is awesome, well done.

Now the small problem I have...... I could not get it to work with my UHF mmdvm hotspot at all, i've set up numerous hotspots / different versions of DMR radio in the past so I am reasonably experienced, after spending a few hours on it, I think I have found the reason why.

Before I updated the firmware, I checked that the radio was exactly on frequency as I had read that tuner adjustments are not currently possible using the GD77 firmware, I checked the radio TX frequency using a HP 53131 counter with external GPS locked reference, both on the A band and B band at VHF and UHF, adjusting using the TYT CPS tuning where necessary to get it correct, hotspots are really critcal with regard to frequency and deviation.
After flashing the GD77 firmware though, the UHF frequency is low by 550 Hz, the VHF is exactly correct.
For analog, 550Hz is not really much of an issue but for DMR, the hotspot will not decode it at all, I then adjusted the hotspot frequency by 550Hz and the radio now works fine through the hotspot, but the hotspot now only works using this radio, not my Motorola / Anytone.

The radio is a version 2 hardware, and I am using the latest OpenGD77 firmware form the page that details the installation instructions (menu says Open MD9600 built 15:45:57 Jun 11 2022 cc117fc D)

These are the frequencies I programmed into the radio with the programming software and what the frequency counter said they were.

144.00000 transmits at 145.00000
146.00000 transmits at 146.000000
430.00000 transmits at 429.99945
440.00000 transmits at 439.99945

It doesn't matter if the UHF frequencies are programmed through the software into a memory channel as I did above, or entered via the radio keypad in VFO mode, the offset is always 550Hz low on UHF, so it would appear that it is firmware in the radio rather than the programming software.

I then reverted the firmware back to stock (TYT version MD9600-CSV-GPS-P06.24.bin), the frequencies on UHF are now exactly correct, so 430.00000 transmits on 430.00000 and 440.00000 transmits on 440.00000.

Flashing the OpenGD77 firmware again, the UHF frequencies are low by 550Hz again.

Appreciate the GD77 firmware is still in development, is it possible that some way of adjusting the reference frequency in the radio, like there is for battery cal and temp cal through the menu,or in the programming software could be implemented, or have I possibly found a 'bug' in the firmware when setting the PLL for UHF frequencies?

Any help with this is much appreciated, I have access to high grade calibrated test equipment should it be required to pinpoint the cause, and do any testing required to get UHF back on frequency.

Regards
Tony G1HMO

g1hmo
Posts: 4
Joined: Thu Aug 11, 2022 3:18 am
Location: Staffordshire, UK

Re: UHF Frequency problem.

Post by g1hmo » Thu Aug 11, 2022 4:14 am

Further bit of info,
I have put the hotspot back exactly on frequency again, it now will not decode the MD-9600.
If I edit the tx and rx frequencies on the radio manually through the menu / mic keypad, so they are high for example, I have set the hotspot to 439.13750.
If I then edit the frequency in the channel using the mic keys to be 439.13805, then the transmitted frequency displayed on the counter is exactly correct, the hotspot will now decode the radio perfectly with around 0.1% BER.
This will provide a temporary 'fix' but the problem now is that if I read the radio with the GD77 programming software, the frequency seems to get reverted back to 439.1375 in the programming software, so every time I read and write the radio, the manual adjustment will need to be done again for each channel!

Tony
G1HMO

G4EML
Posts: 919
Joined: Sat Nov 16, 2019 10:01 am

Re: UHF Frequency problem.

Post by G4EML » Thu Aug 11, 2022 8:58 am

The MD9600 has separate reference oscillators for UHF and VHF. The frequency of each is controlled by the sum of several signals. The bias point of the modulator, the bias point of the CTCSS encoder and a bias voltage from a potentiometer.
We have made efforts to get the frequency calibration as close as possible to the official firmware but differences in the code and in the hardware of different radios means slight errors may still occur.

Luckily the radio frequency alignment is done the 'old fashioned' way using potentiometers. So if you have the correct test equipment you can easily adjust it.

Remove the top cover of the radio. You will see two small preset potentiometers on the edge of the board nearest the front panel. The one nearest the ribbon cable is for UHF, the one furthest from the ribbon cable is for VHF. These should be adjusted to give the correct transmit frequencies. The receive frequencies will then also be correct.

Colin G4EML

g1hmo
Posts: 4
Joined: Thu Aug 11, 2022 3:18 am
Location: Staffordshire, UK

Re: UHF Frequency problem.

Post by g1hmo » Thu Aug 11, 2022 9:16 am

Morning Colin,

Many thanks for the reply, i've just tweaked the UHF pot and got the transmit frequency spot on, it now works with my hotspot with the correct frequencies programmed in both radio and hotspot.

Odd that the stock MD9600 firmware didn't require any adjustment though, but it is now sorted, I can spend this afternoon figuring out how to drive the new firmware.

Thank you again

73's
Tony G1HMO

G4EML
Posts: 919
Joined: Sat Nov 16, 2019 10:01 am

Re: UHF Frequency problem.

Post by G4EML » Thu Aug 11, 2022 9:23 am

I just noticed that you said you used the TYT tuning in the original CPS to adjust the frequencies.
That may be where the difference is occuring. The factory never changes the reference oscillators tuning values. They are always set to the same value for both bands in all radios. (From memory I think it is 150). It appears the factory adjusts the frequency using the pots as previously described.

Colin

g1hmo
Posts: 4
Joined: Thu Aug 11, 2022 3:18 am
Location: Staffordshire, UK

Re: UHF Frequency problem.

Post by g1hmo » Thu Aug 11, 2022 11:23 am

Hi Colin,
Yes you are correct, I have just checked in my notes I made at the time, the setting was 150 for both VHF and UHF, I didn't touch the VHF one, but the UHF frequency was was high from memory when I first got the radio, I adjusted it using the CPS tuning, (as I didn't know there was any other way!) to get it spot on, according to my notes, I moved it to 161 it then read exactly what the programmed frequency was on UHF.
It has been like that since I had the radio, but 'wrong' since doing the OpenGD77 firmware update, but it sounds very much like you are suggesting, that I should have left the setting in CPS at 150 and tweaked the pot right at the start.
I would have done that had I known, but it doesn't really matter now, the end result is the same, if for any reason I have to revert back to the stock firmware, I may well decide to set the menu item back to 150 and tweak the pot back.

73's
Tony G1HMO

G4EML
Posts: 919
Joined: Sat Nov 16, 2019 10:01 am

Re: UHF Frequency problem.

Post by G4EML » Mon Oct 24, 2022 7:37 pm

The GD77 can be tuned using the CPS. You can access the calibration menu from the extras menu.

The Reference oscillator can then be tuned. You will need a frequency counter or well calibrated receiver to adjust this.

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