The new WB7FID repeater:

The 70 cm Receiver System: Design Philosophy

• TV/Cable Tuners - These days, television tuners are better than ever: They are really quite sensitive (they often have noise figures lower than 5db) and being cable-ready, they will tune in the 70cm ATV frequencies with no little or no modification. Integrating one of these into a repeater can be tricky, though: How does one permanently "park" it on the repeater input so that it always comes up on the correct frequency? If you are running 434 or 426.25MHz then what about that difference between "normal" cable channel frequencies and the actual frequency? Another consideration is that while these tuners may indeed be sensitive they do not have very good dynamic range.
• ATV-Specific downconverters - These are downconverters made specifically for ATV use. Typically, these convert from the ATV channel to a low VHF television channel (typically 2, 3 or 4.) Some are only slightly better than some TV tuners. Most have free-running VFOs for their local oscillator so the user must make sure that things are properly tuned in. Of course, for repeater use, you'd be crazy not to have that VFO frequency-locked or have it crystal-controlled. I have had the opportunity to use some of the better-known downconverters of the various makes (no, I won't mention any names...!) and I have been disappointed. Most of them have little or no front-end filtering, poor dynamic range, and are thus susceptible to image response and overload from broadcast TV. The latter is the most serious problem in this area and it is compounded in Salt Lake City area by the fact that there is a local channel 14 that is in the same direction (and polarization) as the repeater. Many ATV viewers have been frustrated by overload from this UHF broadcasting station. This is a fixable problem, but it is frustrating for those just starting out. It is my opinion that in terms of overall performance, the most popular of converters work more poorly than tuners on even inexpensive televisions mainly because of the lack of pre-mixer filtering.

On the previous incarnation of the WB7FID ATV repeater we used a modified UHF varactor tuner. Several steps were taken to make it more useable for use in an ATV repeater (and an inband ATV repeater at that!)

• The Local Oscillator was locked to the correct frequency. A sampling loop was added to the tuner can fed to a PLL. We did have a bypass switch on the receiver so that we could still use the old voltage tuning - sometimes it was useful to look at a commercial TV station to verify operation, check levels, etc.
• The original tuner had a 300 ohm twinlead input. This was changed to a coaxial 50-75 ohm input. The use of a balanced line input (even with a balun!) is not recommended since shielding and isolation really is important in this application and good balance is difficult to maintain. Keep in mind that this was an inband machine, with the transmitter just a couple of feet away!
• Good preamplifiers. There were two preamplifiers in the system on this repeater. One was a tower-mounted preamplifier located at the receive antennas. Integral to the preamplifier was a two-pole bandpass filter, 6-7 MHz wide at the 1db points, centered on the input frequency. At the output frequency it had 15-20db of attenuation. This GaAsFET amplifier had suitable dynamic range to prevent it being overloaded by the transmitter (and the filters didn't hurt either...)
• Good receive filters. At the bottom of the receive coax there was a 5 pole interdigital filter. This had at least 30-40db of attenuation at the transmit frequency. Following this filter was the other GaAsFET preamplifier providing just enough gain to allow the AGC of the receiver system to act on the system noise.

The 70 cm to IF downconverter:

Secondary bandpass filter with secondary GaAsFET amplifier, the High-Level mixer, IF amplifier, and IF lowpass filter Note:  Shielding has been removed to allow a better view.

Another look at the High-Level mixer, IF amplifier, and IF lowpass filter

The conversion of the RF to IF frequencies should be done with low added noise and distortion. The above-pictured input filter is a 2-pole bandpass filter with less 0.5db of ripple and low insertion loss.  It is constructed of UT-141 semi-rigid coax and a GaAsFET preamplifier, based on an MGF1302, is integral to its second pole. Its job is to further remove off-frequency signals and any conversion images (including image noise) from the first preamplifier.

This filter has 1db, 3db, 10db, and 20db bandwidths of 11.3, 14.7, 22.9, and 40.6 MHz, respectively, and is centered at 439.15 MHz. The relatively wide bandwidth is partially intentional: A narrower bandwidth would increase the insertion loss, and the wider bandwidth allows the receiver to be tuned to 434 MHz or 439.25 MHz LSB, if it is so-desired. The plot, by the way, is taken from the IF output port with the converter tuned to receive the 439.25 MHz input. The scale is 2db/div in the vertical and 5 MHz/div on the horizontal axis.
 

Passband sweep of the receiver converter at the IF output terminal.  This sweep characterizes the 2-pole bandpass filter pictured above.

The MGF1302 preamplifier is set up with a high bias current (to improve dynamic range) and it amplifies the signal further to minimize the effects of the mixer loss. The mixer is a Mini-Circuits TUF-2H high-level diode-ring mixer followed by a simple diplexer and a MAV-11 MMIC as the post-mixer amplifier. Finally, there is a 75 MHz lowpass filter to get rid of the mixer image and any LO bleedthrough.
 

Local oscillator, LO Amp, and Prescaler

You might guess that the Local Oscillator had to figure in here somewhere. This converter uses the Mini-Circuits POS-535 VCO. This VCO can tune from the mid 300 MHz range to the mid 500 MHz range. It has a reasonably low phase noise and an output level capable of driving a mixer directly... but not this high-level mixer. A MAV-11 MMIC is used to take the +8dbm (typical) output level of the VCO to the 50+ milliwatt level required by the mixer. Also shown in the picture is the divide-by 64/65 Dual-Modulus prescaler, an MC12032AP, which is an integral part of the synthesizer chain.

The controller provides a dual-modulus programmable divider and the synthesizer reference reference The synthesizer is controlled by a PIC microcontroller that directly programs the dual-modulus synthesizer (in 62.5 KHz steps) for the appropriate LO frequency and monitors and reports the synthesizer status to a secondary repeater controller via an RS-485 port. The secondary controller can then report this to the main controller. The PIC processor has on-board EEPROM (non-volatile) memory and can store default frequencies and operate as a stand-alone unit with no external control, a number of preset "channels" and other parameters. This is useful in the event that the receiver needs to be remotely re-tuned to provide a better fit of the passband of the received signals in the filters and/or escape some potential adjacent-channel interference.
 

Controller/Synthesizer board

The Video and Audio demodulator:

AM Audio/Video Demodulator board (left) and electronic attenuator/amp board (right)

It would only make sense to use one of these demodulators, and we do. Prior to being scavenged from a dead VCR, the signal leads and power supply parameters were carefully measured and noted and it has been translated to its own die-cast box. The RF-AGC is output to the downconverter, and the audio and video outputs are routed to the appropriate IFL video and audio modulators to be sent to the transmitter site. The picture is of the incomplete demodulator undergoing some testing.

Yet another box...

The up/down converter section of the IF processor module

There is yet another box in the receive system: The IF filter module.  This module provides several receive system enhancements:

• Additional stopband rejection.  Typical consumer-grade demodulators contain SAW (Surface Accoustic Wave) filters that have an ultimate stopband attenuation of only 30db.  This module contains filtering that increases this by at least 20db.
• This module provides the ability to put notches in various places of the video passband.  This allows rejection of signals from FM signals that may be in the passband and it prevents them from "keying" the receiver's AGC.
• Additional filters to provide selective "enhancements" to the receive system.  For example, a +- 250 KHz bandpass filter allows one to view a large ID when a signal would otherwise be undetectable.  This can be done only at the expense of resolution, of course.

Keep watching this page, as it will be updated as time goes on...Do you have any questions/comments about what you have just read? If so, please email me and make an ask of yourself...

This page last updated on 20000814

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