900mhz

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Kb8ZGL
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Joined: Tue Apr 24, 2018 1:20 pm

900mhz

Post by Kb8ZGL » Sun Feb 17, 2019 1:41 pm

Does the ZumSpot actually work on 900mhz? Any of the versions? I have not been able to get it to work, it takes the frequency, but doesn't seem to actually hear or TX RF?
Mike
kb8zgl

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VK7HSE
Posts: 96
Joined: Sun Apr 15, 2018 6:35 pm
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Re: 900mhz

Post by VK7HSE » Mon Feb 18, 2019 7:01 am

The ADF7021 RF chip is certainly capable of transmitting there, but not sure how it would work with pistar (don't think there's any code to limit its use)

Code: Select all

The transmit section contains dual voltage controlled oscillators 
(VCOs) and a low noise fractional-N PLL with an output resolution 
of <1 ppm. The ADF7021 has a VCO using an internal LC tank 
(431 MHz to 475 MHz, 862 MHz to 950 MHz) and a VCO using
an external inductor as part of its tank circuit (80 MHz to 
650 MHz). The dual VCO design allows dual-band operation 
where the user can transmit and/or receive at any frequency 
supported by the internal inductor VCO and can also transmit 
and/or receive at a particular frequency band supported by the 
external inductor VCO.
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Kb8ZGL
Posts: 6
Joined: Tue Apr 24, 2018 1:20 pm

Re: 900mhz

Post by Kb8ZGL » Tue Feb 19, 2019 10:06 pm

Thanks, I'll check it out. I was able to enter a 900mhz frequency, it says it is transmitting, but I can't seem to hear it or get into it.
Mike

K9EQ
Posts: 12
Joined: Wed Jan 09, 2019 4:07 am

Re: 900mhz

Post by K9EQ » Tue Feb 19, 2019 11:51 pm

It's not as simple as changing the frequency. In going from the 440 MHz to the 960 MHz bands, the 7021 needs to be configured differently in order to achieve optimal VCO/PLL performance. I don't believe the MMDVM firmware considers frequency when setting these parameters thus as you move out of the 430-450 MHz area, performance will deteriorate. It may stop operating at some point. You may want to try "inching? your way up to 960. I.e., Does it work at 500 MHz? If yes, 600 MHz?

The other factor probably doesn't apply to you because you hear NOTHING, is the matching network between the 7021 and the antenna. I don't know the specifics of the matching network used on these boards, but there is usually a low-pass filter. The implication being that stuff higher in frequency will have greater attenuation. Any transmitter needs to have a low-pass filter as there can be quite a bit of harmonic radiation.

If you really want 900 to work, I'm sure it can be done. Start by looking up the data sheet for the ADF7021 which is the RF transceiver chip used on most of the hotspots.

73,
Chris, K9EQ

Kb8ZGL
Posts: 6
Joined: Tue Apr 24, 2018 1:20 pm

Re: 900mhz

Post by Kb8ZGL » Wed Feb 20, 2019 12:13 pm

Thanks Chris, getting a hair over my head, but I'll get that info and start reviewing.
Mike

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