Output Attenuator

Introduction

It is possible to add a programmable output level attenuator to the Portsdown transmitter. As of software version 201801061, the output attenuator can be used to adjust the output level of the Portsdown Signal Generator. It is planned that a future software version will allow programmable output levels to be set for each band (and possibly for additional transverter outputs).

Attenuator Types

Attenuators based on the PE4302, PE4312, HMC1119 PE43703 or PE43713 are supported. The PE4302 and PE43703 types are readily available on eBay for under £20. The preferred type is the PE43703 illustrated here:

Data sheets are available at these links: http://www.psemi.com/pdf/datasheets/pe4302ds.pdf http://www.psemi.com/pdf/datasheets/pe43703ds.pdf http://www.analog.com/media/en/technical-documentation/data-sheets/HMC1119.pdf.

Preparation

As supplied, the eBay pre-built attenuator boards are generally configured for control by DIP switches, not by a Raspberry Pi. NOTE THAT IF CONNECTED WITHOUT MODIFICATION THEY MAY KILL YOUR RASPBERRY PI! The modification details for one of the PE43703 boards are detailed here - your board may differ! Please could a PE4302 owner publish similar information??

There are 3 signal lines used to program the PE43703: LE (Latch Enable), CLK (Clock) and Data. For DIP switch programming the LE line is connected to +5v. This line needs to be disconnected from +5v otherwise it will try to put 5v on a 3.3v RPi GPIO output. Referring to the picture below (pre-modification):

The solder blob at J7 connects the LE line to +5v. This blob needs to be sucked or solder-wicked away (taking the usual anti-static precautions). Check with a test meter that there is no connection between the 2 halves.

There is also a parallel/serial switch line that is initially set to parallel. To set it to serial (0v), remove the blob at J4, and create a new blob at J3. Check that J4 has really been disconnected! The top picture on this page shows the unit after modification.

Apply power (5v) to the unit. If all is well, the power LED should light; if not, recheck J4. Then check the voltage on the DATA, CLK and LE pins. They should all be at 0v. Using a 1K resistor, check that each of them can be pulled up to 5v. Only after these checks is it safe to start connections to the RPi.

Daata Connections to the Raspberry PI

The attenuator will share the clock and data lines used by the ADF4351 synthesizer. A separate LE line is used, connected to Pin 10 of the GPIO connector. This pin can be accessed from the "DigiThin" connector on the Portsdown GPIO board.

During testing it has been found that connecting the attenuator board can cause the ADF4351 to fail to start; it seems to be particularly fussy about the rise times on its programming signals. To overcome this problem, and to prevent any possible damage to the Raspberry Pi from the +5v present on the attenuator board, it is recommended that the 3 signal lines to the attenuator are connected through 1K resistors. The connections are as follows: