gd77 autogenerates signal at 430.075 Mhz

Post Reply
User avatar
IU4LEG
Posts: 191
Joined: Wed Nov 20, 2019 12:49 pm

gd77 autogenerates signal at 430.075 Mhz

Post by IU4LEG » Wed Jun 29, 2022 2:45 pm

hi all,
I state that this is not a firmware bug because the defect was also found with the original firmware.
It would appear to be something generated by Hardware.
On 4 gd77 tested I found that a signal is generated exactly at 430.075 Mhz.
In practice there is always a fixed carrier at 4-5 of rssi.
With the external antenna it decreases but does not cancel.
This can disturb the rx of any repeaters on that frequency.
I have tried with other radios like Anytone 878, TyT Md-380 in the same place as the gd77 and on this radio the signal do not exist.
approaching one of these radios to the gd77 the signal also appears on these devices.
Can you confirm that this also happens on your gd77?

Regards,
Fabio IU4LEG

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

Re: gd77 autogenerates signal at 430.075 Mhz

Post by G4EML » Wed Jun 29, 2022 5:05 pm

Yes, it will happen on all GD77s. The actual strength of the signal will vary between radios.

The theoretical frequency is actually 430.080 MHz but again this will vary slightly for each radio.

The crystal clock for the processor and DMR encoder is 12.288 MHz. There will be spurious signals at all harmonics of this frequency and there is nothing that can be done in firmware to prevent this.

12.288 x 35 = 430.080

There is also a second crystal at 26.0 MHz. This will also produce some spurious signals.

More expensive radios often have the ability to slightly shift the crystal frequencies to avoid such problems but the GD77 does not have the hardware to do this.

Colin G4EML

IZ2EIB
Posts: 161
Joined: Sat Nov 30, 2019 12:55 pm

Re: gd77 autogenerates signal at 430.075 Mhz

Post by IZ2EIB » Wed Jun 29, 2022 8:51 pm

Thanks Fabio and Colin, very interesting.
Honestly I had not noticed it.
Even if it is not necessary, I confirm that more expensive transceivers often have the ability to slightly shift the crystal frequencies to avoid such problem, for example digital C4FM handheld FT1, FT3 and FT5 by Yaesu.

73 best regards de Fabio IZ2EIB

ok1pt
Posts: 167
Joined: Mon Jul 20, 2020 3:38 am

Re: gd77 autogenerates signal at 430.075 Mhz

Post by ok1pt » Thu Jun 30, 2022 4:56 am

Hi!
These "autogenerated signals" :-) are obviously known as "birdies" and they are very common on radios with digital circuits inside - like CPU/MCU, various frequency synthesizers, displays, GPS receivers etc. You can find them on every modern radio with wide reception bands including such valued brands like Kenwood, Yaesu, ICOM etc. It's just normal and for some models, these frequencies are listed in their manuals with a short explanation.
Even purely analog radios can have birdies, depending on their frequency plans (obviously they are two- or even three- stage heterodynes so there are local oscillators, which can have some harmonic frequencies passing to the received bands).
With regards / 73,
Pavel OK1PT

User avatar
IU4LEG
Posts: 191
Joined: Wed Nov 20, 2019 12:49 pm

Re: gd77 autogenerates signal at 430.075 Mhz

Post by IU4LEG » Fri Jul 01, 2022 11:13 am

thanks guys for the explanation!

73, Iu4leg

Post Reply