[TenTec] Re: Roofing Filters and DSP BW

[Bill Tippett]

         Regarding Ten Tec filters, for any that are interested I did find 
the following comments in the documentation for the CDG2000 receiver 
project at:

  http://www.warc.org.uk/cdg2000/post%20mixer%20amp%20and%20filters/Part%203%20post%20mixer%20amplifier%20V4.doc
Components

The prototypes used filters manufactured by IQD, type 90H2.4B for the wide 
filter and 91H250 for the narrow filter. Testing on the prototypes yielded 
a very high IP3 for both filters with the CW filter surprisingly good at 
over +45 dBm. These are no longer manufactured although JAB [1] may still 
have limited supplies of the wide filter. A search of the Internet revealed 
two sources of 9 MHz filters, the first was found at Ten-Tec [2]. These 
ladder filters are available at several 6dB bandwidths, namely 2400, 1800, 
500 and 250 Hz. The input and output impedances are both 200 ohms compared 
with 500 ohms for the IQD filters, and the matching networks will have to 
be changed. We tried the model 220 for 2400 Hz and model 217 for 500 
Hz.  The insertion loss is 2.2dB for the 2400Hz filter and 7.5 dB for the 
narrow. These losses are lower than the IQD filters by about 2 dB but the 
intercept point is not as good. Also, the centre frequency was slightly 
higher than 9 MHz and did not match the bandpass of the roofing filters. 
The obvious conclusion is that if these filters are used then similar 
filters (type 220) could also be used for the roofing filters on the front 
end board although this would seriously degrade the IP3 capability of the 
receiver. The second source was found at International Radio Corporation 
[3]. This firm manufacture replacement filters for most Amateur radios 
which are claimed to have superior performance to those originally 
supplied. We would suggest the use of reference 2310 for the wide filter 
and reference 2304 for the narrow. These filters have the same Zin / Zout 
as the IQD filters originally used and should need no change to the design. 
Failing the use of commercial filters, home made filters can be used. As a 
start, the roofing filters used in the front end can be copied and modified 
to cover the relevant passband. International Radio Corporation also supply 
kits for 9 MHz filters and we have obtained two of these with satisfactory, 
if not outstanding results. These are reference 350 and 351 which are each 
4 pole. Two of these can be wired in series to produce 8 pole filters. The 
Zin / Zout is 200 ohms and some slight modification to the matching 
circuits would have to be made. The cost of the kits is roughly half that 
of manufactured filters. If DSP is to be incorporated, then the CW filter 
is perhaps not essential but is still desirable.




References



[1] JAB P.O. Box 5774, Birmingham B44 8PJ, telephone +44 (0)121 682 7045

[2] <http://www.tentec.com/>http://www.tentec.com

[3] http://www.qth.com/inrad

Still looking!

Bill  W4ZV