PANADAPTER
Updated 1st December, 2010, see end of page for a working Panadapter !
A "Panadapter" is a device which is connected to a receiver's IF, at a point before any roofing filters. It may then be used to display signals near those to which the receiver is tuned. Of course, a suitable display is required. In the older days of Panadapters, long persistence display tubes were used. In these times, the display may be via a PC or for a self contained unit, some type of graphical display.The horizontal axis is frequency (centred on the receiver's current frequency) and the vertical axis is signal strength, normally calibrated in some fashion. A Panadapter is a form of spectrum analyzer.
These days, there are quite a few small graphical displays available on Ebay. In particular, the 128 x 64 dot type are very cheap. Under US$10 if you look around. This is the one I bought, a "Lumex S12864 G". This one has in-built LED backlighting, but more importantly, has its own bias supply. Some displays need external 12 or 13V etc. This one only requires a single 5v supply. It is a little port hungry on a Microprocessor interface though, requiring about 12 lines.
I wrote a quick bit of code to try it out. This represents several signals either side of the centre frequency. At the bottom is a scale with centre frequency and +- 10, 20KHz. This is not the final version. Since there are 128 dots across and 64 down, I have allocated a section to the left (dotted line) of 8 dots across for a scale (not there yet) of signal strength with the bottom 8 dots vertically, being the frequency offset.
I may not use the vertical bar idea but use a single dot for each horizontal point, forming the more usual graphical display for a Panadapter. The exact scaling on the bottom is yet to be determined and is limited by the crystal filter used.
WHERE I WILL USE IT:-
I happen to be be building a PICASTAR transceiver. (See Yahoo groups) So I thought of adding a Panadapter display for it. My thoughts are for the IF signal at 10.695Mhz to be buffered out to a point where, say, an old 10.7MHz 25Khz wide filter could be used. (I have since bought a couple of 10.7MHz, 35KHz wide, 8 pole fiters on ebay for AUS$16. Perfect.) Mix this down to maybe 6MHz where cheap crystals are available for a filter to get 200Hz resolution. The filtered output signal can be measured by an AD8307 log detector (or use an FM IF chip with RSSI {signal strength} like the NE604, which is cheaper and easier to get) The local oscillator for the mix down to 6MHz could be done with a DDS controlled by a micro, which sweeps the DDS across the bandwidth of the crystal filter. The micro would convert the level from the log detector using its in-built analogue to digital convertor and then display it on the graphics display.
Here is the bottom of my PICASTAR. Not too tidy just yet as it will be moved into a new homemade box, similar but a little larger than this home made one. The mixer (known as Magic Roundabout) is located at the bottom. The Panadapter will interface to it with a buffer board. Centre is the IF board and above, the T/R timer board.
All of this was prompted by a recent Panadapter article in QEX (I think March/April 2007) and I am about to try an adaptation of this article, using parts to hand, specifically the 128x64 dot graphical display, driven with a Motorola MC68HC908GP32 or QB8 micro. The Panadapter will be self contained, with a (buffered) connection from the PICASTAR IF board, before the 10.695MHz (6KHz wide) roofing filter.
The first thing required is a buffer stage to tap off the IF signal. This should not load the IF stages and it should also provide a low impedance output, for the following stages in the Panadapter. I was looking around the WWW and saw an article on using FET's as source followers. Ref "FET Principles and Circuits, Nuts & Volts Magazine (June 2000), Figure 8." I breadboarded up this circuit for testing: It's not a single FET. It also has a bipolar transistor in the biasing and output stages. It seemed OK, so I etched a PCB for it.
I used BC547's
in prototype, but 2N3904's in PCB version.
If this buffer proves OK with low signal levels, I will fit it to my PICASTAR, with a co-ax cable and connector on the rear. A shield will be fitted around the PCB. At this stage, the Panadapter will probably be external, but if I re-build my PICASTAR in a new case, I may incorporate it also.
Here is the information for the PCB in pdf format. It's given as is. I have not done any further testing at this stage.
Following the buffer, some amplification may be needed, followed by a mixer, to mix down to the 6MHz filters.. I will use a SBL type mixer initially as its quick and easy. Later I may go to an H mode mixer, as is used in PICASTAR.
More later.........
A friend (Peter) has taken up the Panadapter project and now has a working prototype. Very impressive result.
A 128x64 graphic display is used with an Atmel controller. Sweep rate seems very fast so updates are quick.
Below are some pictures of actual on-air signals. There is some 'ghosting' due to the fast sweep and slow camera !
The sweep width is 200KHz......
Above: currently only a 1KHz home made filter is fitted. A 200Hz will be added later.
I will add an IF output on my STAR and try Peters Panadapter in the near future. <vk3pe>
Page created by VK3PE
Updated:- December 1, 2010