GD-77 sensitivity on VHF, how can I calibrate the radio ?

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VK1EA
Posts: 5
Joined: Sun May 29, 2022 12:34 pm

GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by VK1EA » Sun May 14, 2023 3:21 am

Hi all,

I have a GD-77 I have been enjoying for some time thanks to the OpenGD77 firmware. All DMR activity here in Canberra takes place on UHF.

However, I recently tried to use the radio in VHF FM, and noticed that the sensitivity was nowhere as good as the other VHF analog radios I had at home (both mobile and portable ones), from ear it is more than 10 dB worse. This is not related to the NFM or WFM selection or the antenna used.

Is there a way to adjust the frontend bandpass filters? I have access to an RF generator and a SINAD meter.


Many thanks in advance,
Carlos VK1EA

VK3KYY
Posts: 7486
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by VK3KYY » Sun May 14, 2023 3:37 am

VK1EA wrote:
Sun May 14, 2023 3:21 am
Hi all,

I have a GD-77 I have been enjoying for some time thanks to the OpenGD77 firmware. All DMR activity here in Canberra takes place on UHF.

However, I recently tried to use the radio in VHF FM, and noticed that the sensitivity was nowhere as good as the other VHF analog radios I had at home (both mobile and portable ones), from ear it is more than 10 dB worse. This is not related to the NFM or WFM selection or the antenna used.

Is there a way to adjust the frontend bandpass filters? I have access to an RF generator and a SINAD meter.


Many thanks in advance,
Carlos VK1EA
No. There are no software or hardware adjustable filters in the radio.

BTW. Generally the gain on VHF is OK, so you may have a hardware fault on your radio.

The other potential problem is very strong QRM regardless of what frequency, as the RF chip uses in these radios is renound for not handling strong signal interferance.

e.g. If you are close to a broadcast radio transmitter on 100MHz, this will desensitise the radio, as will being in proximity to TV transmissions on 200Mhz etc

Some people modify their radio to fit internal shielding inside the plastic case, but this won't totally overcome the cheapness of the RF chip, which literally costs $1.

VK1EA
Posts: 5
Joined: Sun May 29, 2022 12:34 pm

Re: GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by VK1EA » Sun May 14, 2023 4:02 am

Thank you Roger,

I found a copy of Jason's work on the calibration "softpots" but there are lots of values he marked as "unknown", I was hoping that maybe someone managed to identify those related to the receiver's frontend.

https://nq4t.com/anytone/GD-77tune.htm



73,
Carlos VK1EA

VK3KYY
Posts: 7486
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by VK3KYY » Sun May 14, 2023 6:49 am

VK1EA wrote:
Sun May 14, 2023 4:02 am
Thank you Roger,

I found a copy of Jason's work on the calibration "softpots" but there are lots of values he marked as "unknown", I was hoping that maybe someone managed to identify those related to the receiver's frontend.

https://nq4t.com/anytone/GD-77tune.htm



73,
Carlos VK1EA
The PCB and all the hardware have been examined in great detail and are no CPU controllable RF hardware elements on the radio apart from inside the AT1846S transceiver chip.

Also all the data that gets sent to the RF chip by the official firmwar has been analysed and there are only a few values which are undocumented in the available data sheets.

Those undocumented values are unlikely to fix the problem you have.

VK1EA
Posts: 5
Joined: Sun May 29, 2022 12:34 pm

Re: GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by VK1EA » Sun May 14, 2023 8:50 am

Well,

It seems to be the way it is, I found several people commenting about sensitivity issues at the RadioddityGD77 facebook group.

Is there a proper datasheet available? I cannot find any, only TX figures from the FCC certification report.

73,
Carlos VK1EA

VK3KYY
Posts: 7486
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by VK3KYY » Mon May 15, 2023 10:14 am

VK1EA wrote:
Sun May 14, 2023 8:50 am
Well,

It seems to be the way it is, I found several people commenting about sensitivity issues at the RadioddityGD77 facebook group.

Is there a proper datasheet available? I cannot find any, only TX figures from the FCC certification report.

73,
Carlos VK1EA
Not sure what you mean by 'proper datasheet', but I think the answer is No.
The AT1846S is a Chinese made IC and they don't publically release the data sheet. But there are various partial data sheets kicking around on the web, none of which contain complete information.
The same, or possibly a similar chip is also know as the RDA1846, but its not clear whether this is exactly the same as the AT1846S or not.

Most of the information about this chip is limited to what its internal control registers do. I've never see much relating to its RF operation and performance.

VK1EA
Posts: 5
Joined: Sun May 29, 2022 12:34 pm

Re: GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by VK1EA » Mon May 15, 2023 10:42 am

Hi Roger,

By a "proper datasheet" in the context of the discussion on VHF sensitivity, I mean minimum system sensitivity (not the chip alone) for a certain SINAD across the VHF range the manufacturer claims to cover. Manufacturers state the testing conditions for you to replicate the test (like the FM deviation used or the frequency of the audio test tone).

Unfortunately I could not find that, only marketing statements. Nothing about the receiver sensitivity at the FCC either.

Jason made some sensitivity measurements while investigating the ability to operate in 222 MHz, but the lowest he went (or published) was (from memory) 190 MHz, and it was around -95 dBm (if I remember correctly).

I'll see if I can find someone with a GD77 here in Canberra. While I don't have much testing gear at home, I have a CMA-180 at the office I can play with.

If someone has measured VHF sensitivity, or knows of where to find other people's measurements or data, please let me know. Not really interested about other RX characteristics like desensitization due to strong fields, blocking, RX IM3, parasitic responses and the like. Only old-fashioned FM sensitivity.

Many thanks in advance.
73,
Carlos VK1EA

G4EML
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Re: GD-77 sensitivity on VHF, how can I calibrate the radio ?

Post by G4EML » Mon May 15, 2023 12:19 pm

From memory, the performance of the GD77 is pretty reasonable on both bands and compares well against other radios. Certainly not 10dB down on VHF as you are suggesting.

It is a while since I tested my GD77 so I have just repeated the test on my Marconi 2955.

The performance is identical on VHF and UHF and doesn't change by more than a dB across the whole frequency range.

FM 3KHz deviation. 12dB SINAD at -124dBm. 20dB SINAD at -121dBm.
FMN 1.5Khz deviation 12dB SINAD at -124dBm. 20dB SINAD at -121dBm.

If you are getting significantly worse figures on VHF than UHF then you may have a faulty VHF front end. The radio has completely separate RF stages for VHF and UHF.

As mentioned by Roger, on VHF the radio does suffer from overload and blocking by out of band signals so its performance on an external antenna can be quite poor.

Colin G4EML

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