Yaesu VX-7 Technical Review

The Yaesu VX-7 main receiver covers:

  • 0.5 - 728.995 MHz
  • 758.000 - 773.995 MHz
  • 803.000 - 823.995 MHz
  • 849.000 - 868.995 MHz
  • 894.000 - 914.995 MHz
  • 944.000 - 959.995 MHz
  • 989.000 - 998.995 MHz

Why the gaps right at the most useful frequencies? Because of FCC R&O 99-58, 38 dB of rejection for 12 dB SINAD signal in the Cellular Block A and Block B bands. This was an update to FCC ET Docket 93-1 establishing the obligation of OEMs to block Cellular Radio Service (824-849, 869-894 MHz) reception after some embarrassing VIP eavesdropping scandals.

OEMs may block image frequencies such as the 902-928 MHz band that would be very useful for amateur radio operators to receive on. The Yaesu VX-7 has a fourth-order varactor-tuned bandpass filter comprised of HVC355B varactor diodes (D1015 and D1017) with parallel inductor tank circuit for each section of the filter. The IF frequency is 47.25 MHz, and Yaesu is blocking the first and second image frequencies, hinting that the Q of the filter is low–very broadband.

IF image matrix vis Yaesu’s frequency blocking strategy:

  • 824-2*47.25-0.45=729.05 MHz ← right where Yaesu starts blocking frequencies.
  • 849-2*47.25-0.45=754.05 MHz ← OK, that’s blocked, but they wait until 758 MHz to start allowing reception. Design fluke?
  • 869-2*47.25-0.45=774.05 ← they start blocking again
  • 894-247.25-0.45=779.05, 849-47.25+2 0.45=802.65 ← I guess they rounded up to 803 MHz
  • 869+47.25-2*0.45=915.35 ← rounding down to 915 MHz I guess to start blocking
  • 849+2*47.25+0.45=943.95
  • 869+247.25-2 0.45=962.4 ← rounding down to 960 MHz?
  • 894+2*47.25+0.45=988.95

So that means it’s the first image that’s an issue–isn’t that what the tunable bandpass filter in the first IF is supposed to be for? Usually we expect 50-70 dB image rejection. Clearly that’s not the case here, hence the frequency gaps.

12.5 kHz channel compatibility: the VX-7 has filters only for 25 kHz channels. The transmit deviation can be reduced, but the receiver will still be vulnerable.

Split PL/DPL tone: the VX-7 can only do a mix of PL/DPL split, you can’t do distinct TX/RX analog PL tones. Some repeaters with high altitude and/or voting receivers use DPL on the repeater receiver input, and transmit with analog PL. This is sensible because repeaters are targeted toward mobiles and portables, and maybe base stations with directional antennas and low power. Both in the commercial and amateur world, base stations with high power omnidirectional transmissions clog up distant repeaters, particularly at VHF where tropospheric ducting is more common.

Why not use DPL for both transmit and receive? Because DPL has worse decode sensitivity than analog PL, particularly for low-quality decoders. The repeater will have a high-quality DPL decoder, and the mobile is unlikely to hear the distant repeater, so analog PL receive is good for the mobiles, why give a double SNR decode penalty with DPL TX/RX.

For a system targeting wide geographic coverage, two alternating repeater RF output frequencies might be used, and keep the same PL for simplicity. The systems can be linked via RF or VoIP backhaul, and the end user selects which repeater receives them by switch mobile transmit PL. If the systems are unlinked, interference is avoided since everyone hears the overlapping repeaters. This is something the VX-7 cannot do, without using PL on transmit and CSQ receive.

Modern cars and environments have a lot of RF noise from computers and communications. Carrier squelch will breakthrough not infrequently on nuisance interference. Repeater should transmit coded squelch so users can filter out background RF noise.

The RX sensitivity specifications are easily met. We hear airplanes a couple hundred km away on VHF AM airband (108-136 MHz).