Lack of 135 to 165 MHz RX on 6500

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  • Updated 6 years ago
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I am guessing that the 6700 uses passband filtering and harmonic subsampling to receive 135 to 165 MHz. Why doesn't the 6500 use the same technique? Lack of room for filters? No room in FPGA? Product feature differentiation?

Also is the 135 to 165 MHz RX available on both SCUs of the 6700? Which ant ports can you use?
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Tim K8XS

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Posted 6 years ago

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Ken - NM9P, Elmer

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My guess is that adding extra receive frequencies would have added enough cost to the 6500 that it would have taken it out of the price point.

But I am curious, just for discussion's sake, about the technology as to would it have been possible to have it on the 6500 and how much additional complexity and price it would have added?

Are there any good designs out there for a simple (inexpensive) receive converter for VHF that would allow the wide-spectrum coverage of the 6000 series to be used on VHF / UHF? (I am aware of classic receive converters, but most of these are pretty narrow band compared to the 6000.)

Ken - NM9P
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Tim K8XS

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The only part that would be required would be a VHF preamp and a good BPF that would pass 135 thru 165 MHz and cut everything else out. Because we would be sampling below Nyquist, the sampled spectrum would fold over into Fs/2. However, this would not cause harmful aliasing because the only signal we would get would be the band that we passed thru the BPF and that is what we wanted. Just a little software magic and there you are. Obviously you might want some relay switches, the VHF preamp, and ...
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George Molnar, KF2T, Elmer

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Tim, are you saying that with a suitable low-noise preamp and filter, one could effectively tune VHF on the 6500 through the auxiliary antenna port?
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Tim K8XS

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George- I am guessing not because I suspect FLEX already has a LPF on all ports leading to the SCU that would cut all signals off above 77 MHz.
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Greg - K5GJ, Elmer

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Official Response

Your post actually has most of the answers in it:

1. The FLEX-6700 and FLEX-6700R are the only radios in the FLEX-6000 family that support the 135MHz to 165MHz band for receive/low level transmit.

2. On the FLEX-6700, we accomplish the VHF band by using a combination of:
a. Sampling above the critical Nyquist frequency.
b. A much larger and faster Field Programmable Gate Array for processing
c. A faster System Processor for processing
d. Dual Stage Pre-amps to make up for the loss associated with this sampling technique.
e. A complex anti-aliasing/bandpass filter to block FM Broadcast signals from overload but allow the VHF signals to pass. Note that on the FLEX-6500, a 70MHz anti-aliasing filter is inline always so no VHF signals will ever make it to the SCU A/D converters..

3. Yes both SCUs on the FLEX-6700 have the capability to receive VHF. Also, the Direct UpConverting (DUC) exciter is configured and filtered to allow for low level VHF transmission of up to +5dBm output power in the 144-148MHz range.

4. Lastly, YES, there is distinct product differentiation between the FLEX-6700 and FLEX-6500 products. The FLEX-6700 was designed as a "NO HOLES BARRED" Best in Class transceiver where we spared no expense to get the top performance. The FLEX-6500 provides much of the same world class performance with a slimmed down feature set to address a specific market point. Because of the additional costs involved, VHF was not included in the FLEX-6500.

I hope this information clears up any confusion.

Greg - K5GJ

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