Linear ATV Repeater


Background:

Television translators have used this approach for repeating signals over a wider area. By removing the process of demodulating and remodulating signals you are able minimise the amount of distortion at each stage of translating. This is why linear amplifiers are so important when it comes to Television, with AM (VSB vestigial side-band) the peak sync is at the point maxim output. This where sync compression is most likely to take place. The process works like this where th RF input mixed down to an IF (38.9 MHz). Where the IF is amplified and filtered. Next the IF is then remixed back to a RF frequency and filtered then amplified.


Introduction:

With ATV changing from analog to digital wouldn't be good to have a repeater that could work with both, this is why I took a new look at linear repeaters. Another advantage is where these types of repeaters can be used as ATV extenders. As in my MKII ATV receiver design using Comtech L-band tuner modules I look at other types of units that were available. I found that they had L-band down and L-band up-converter modules as part of there product range.


Features:

1/ Wide turning 800 to 2200 MHz

2/ 12 volts DC powered

3/ Controlled via RS-232

4/ Bar graph display

5/ 27 MHz IF bandwidth

6/ Switchable controlled LNB volts

7/ Dual mute system

8/ Overload alarm
Circuit description:

The front end is very much like a normal Satellite receiver and can be be used in the same way. Such as adding in a LNB for the microwave band of interest or direct in for 33 cm and 23 cm bands. The L-band converter takes the incoming RF and mixers it down to an IF of 480 MHz, with an IF filters with a bandwidth of 27 MHz. On the DC feed side there is a P-channel power FET this is a switch to turn on and off from the Micro-controller. To find the optimum input level I have used a 74HC164 gate driving eight LED's in a bar graph form. This done by reading the AGC level via A to D on the Micro-controller. There is a single LED bar as shown in the photo, if this bar is visible you are the operating range of the AGC. I have tested the AGC this is the response curve that I work out. As you can see there is a wide input range of -20 to -70 dBm with a corresponding output of -17 dBm.
The dual mutes are set at -68 dBm at the low level and -20 dBm at the high end. This done by switching a LM2941 regulator that can be used to turn on and off a power amplifier stage. I have also provided an alarm output to indicate when the front end is starting to go in to overload.

These converters are arranged around the Mitel PLL chip the SP5055. This PLL is driven with an I2C serial interface that is controlled via a PIC 16F876A Micro-controller. The down converter side is address at hex C0 and up converter has the address of hex C6. The 2 x 16 LCD is not needed for operation, the settings are set by the RS-232 comport at 9k6 baud rate.

I used the same voltage doubler in this circuit as in the MKII receiver using the NE555. The NE555 timer oscillating at 22kHz, this drives two transistors in a Totem pole configuration. This feeds two diodes that provides the required turning volts for both units.

On the L-band up converter the IF is feed in after the saw filter into this module and mixed up. There is also where the AGC out that is feedback to the down converter. The RF output is filtered then amplified to the output SMA connector. ERA-3 and ERA-5 are the driver output amplifiers that bring the drive level up to +15 dBm for the power amplifier stage.


Setup via RS-232:

Using MS Hyper terminal this is what you should see is this message:

RX / TX Converter

by ZL1WTT
R1282

1282 MHz


Set receive frequency [Enter]

Echo

Receive frequency




ON

LNB volts on

Turn on LNB volts

[Enter]

Echo

LNB volts on


V?

Supply 12.5 v

Read supply volts

[Enter]

Echo

Supply volts
T0920

920 MHz


Set transmit frequency

[Enter]

Echo

Transmit frequency


OFF

LNB volts off

Turn off LNB volts

[Enter]

Echo

LNB volts off


S?

-40 dBm

Read RX signal level

[Enter]

Echo

signal level in dBm
WM

Writing data


Write to memory

[Enter]

Echo

Writing data


R?

1282 MHz

Current set RX frequency

[Enter]

Echo

Current set frequency


??


[Enter]

Display all settings

Echo

All settings
M?

RX 1282 MHz

TX 920 MHz

Read memory

[Enter]

Echo

TX and RX frequencies


T?

920 MHz

Current set TX frequency

[Enter]

Echo

Current set frequency
Performance and testing:

I have tested this down converter performance down to the noise floor of about -102 dBm this was without any preamp. Levels below -70 dBm were translated ok but the output level started to drop away at this point. The up converter frequency range I tested from 800 to 2150 MHz without any problems, above this frequency the performance was some what poor. The RF output levels varied across this frequency range from -17 to -25 dBm at the low and high ends. I had no problems converting from the 23 cm band down to the 33 cm with both types modulation, analog FM and digital QPSK.



ATV extender:

An ATV extender can be used to provide infill coverage to areas the main repeater is unable to get in to. Here is example of a full-duplex extender setup using the 23 and 13 cm bands. This is done with two lots of 23 and 13 cm antennas these could be in the form of Loop Yagi's. Minimal filter requirements are needed as both inputs and outputs are on the same band.
Copyright © 2010 by Grant ZL1WTT  ·  All Rights reserved  ·  E-Mail: zl1wtt1@yahoo.com.au