I posted this on the 900MHz Ham Conversion forum, but I've received zero replies. Hopefully, it will be noticed here.
I heard a rumor that it is possible to tickle an old Motorola cell phone from 800 into the 900 MHz ham band.
Any truth to this?
Any tips or tricks?
Thanks!
Motorola AMPS phone to 900 Ham Band?
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Jonathan KC8RYW
- Posts: 1747
- Joined: Fri Dec 28, 2001 4:00 pm
You didn't mention which Motorola mobile phone? The old mobile originals (which cost $5000) are very discrete built similarly to two-way radio. Here are some general points:
You would have to disconnect the duplexer. There are some old NOVATEL phones with screw tuned duplexers. These are bandpass, so if the screws aren't solder fixed, you could retune. Would likely be easier to bypass. Newer duplexers are ceramic... no chance retuning these. Also all are 45 MHz split.
Cell phones are full duplex. The LO often forms the transmit exciter (835 MHz). 45 MHz higher is the receive frequency (880 MHz). This 45 MHz is the IF frequency.
On anything but the oldest phones, the frequencies are hardcoded into the firmware. The only way left to change the frequency is to change the reference oscillator. You will also have to figure out the tech mode where perhaps you can set the TX or RX on a fixed frequency. It won't come up on this frequency automatically when you power up. You will have to enter the test mode again. Else you could build a new PLL to control the VCO.
AMPS is 30 kHz channel spacing with wider deviation than two-way.
HOWEVER, there are some parts that you can use from older mobile phones. Some radios use different IF's with an LO up at 915 MHz. They have a ceramic four pole filter that passes about 910 to 925 MHz. The class C PA is often OK if you want to build your own AMPS enhancer, or even for a 3W 900MHz PA. If you get a couple of old phones, you might be able to put together a full duplex enhancer for your car or cottage. You will be limited by the 50 dB duplexer isolation. Often, the APC is integrated with the PA, which you can also make use of at 900 MHz. You may also find an EPROM for reuse.
Finally, the HANDSETS are actually 300 ot 600 baud asynch TERMINALS that could be reused for an electronics project. Their RJ-45 style jack will have pins for V+, GND, audio in, audio out, data in, keypad out. It isn't that difficult to figure out how to paint numbers onto the display. Press a key, and you get a data burst out of the KEYPAD OUT lines. Etc. Etc.
Good Luck,
RFDude.
You would have to disconnect the duplexer. There are some old NOVATEL phones with screw tuned duplexers. These are bandpass, so if the screws aren't solder fixed, you could retune. Would likely be easier to bypass. Newer duplexers are ceramic... no chance retuning these. Also all are 45 MHz split.
Cell phones are full duplex. The LO often forms the transmit exciter (835 MHz). 45 MHz higher is the receive frequency (880 MHz). This 45 MHz is the IF frequency.
On anything but the oldest phones, the frequencies are hardcoded into the firmware. The only way left to change the frequency is to change the reference oscillator. You will also have to figure out the tech mode where perhaps you can set the TX or RX on a fixed frequency. It won't come up on this frequency automatically when you power up. You will have to enter the test mode again. Else you could build a new PLL to control the VCO.
AMPS is 30 kHz channel spacing with wider deviation than two-way.
HOWEVER, there are some parts that you can use from older mobile phones. Some radios use different IF's with an LO up at 915 MHz. They have a ceramic four pole filter that passes about 910 to 925 MHz. The class C PA is often OK if you want to build your own AMPS enhancer, or even for a 3W 900MHz PA. If you get a couple of old phones, you might be able to put together a full duplex enhancer for your car or cottage. You will be limited by the 50 dB duplexer isolation. Often, the APC is integrated with the PA, which you can also make use of at 900 MHz. You may also find an EPROM for reuse.
Finally, the HANDSETS are actually 300 ot 600 baud asynch TERMINALS that could be reused for an electronics project. Their RJ-45 style jack will have pins for V+, GND, audio in, audio out, data in, keypad out. It isn't that difficult to figure out how to paint numbers onto the display. Press a key, and you get a data burst out of the KEYPAD OUT lines. Etc. Etc.
Good Luck,
RFDude.