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DDS Amplifier

DOUBLE DDS
As I wrote before, signal level of the DDS is not sufficient for IM measurement of passive devices with high IP3.  Signal from the DDS will be amplified in a linear amplifier. Every DDS channel will use separate amplifier, installed in a  Hammond case. I used practically the same amplifier as in the first kit, but without gain control possibility. Output level will be controlled directly from the DDS. For better impedance matching and reverse isolation two 10dB attenuators are used at the input and between first two stages of the aplifiers. Current consumption without signal reaches abt. 700 mA. If necessary, fixed external attenuators will be used at the amplifiers input for the purpose of IM measuring (to set proper rf signal level).


Two layers PCB dimension (88 x32.5mm) is adequate for insertion to the case after corners adjusting.


PCB has been designed for use FB 43-1502 core at the output stage. After first measurement core type was changed to BN 43-202 and diode D3 to ER3A type.


The heat dissipation from Q2 and Q3 is provided by two Alluminium pillars (5mm high) affixed to the Hammond case, with termal foil on the top side, touched PCB from the bottom. Q1 is fixed directly to the case.


For the purpose of first measuring amplifier case was attached to a small cooler.



The following pictures show results of the basic measurements. Gain of the amplifier reached abt. 35dB in the HF range, 33dB in 6m band. VSWR is  excellent  at the  input,  better  than 1.5  at  the  output  (thanks to attenuation used).  I want to repeat again  that   impedance  matching  over   the  full  measurement  chain  is  one  of  the  most  important  parameters  to achieve the correct results during intermodulation measurements. Reverse isolation is not so high as in the first kit, but I  thing it  will be good enough for my purposes.
 
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