WDG042 Switched Attenuator
A high accuracy 5 bit switched attenuator is under development and we are hoping to beta test this design in the near future. Please email G3WDG if you are interested in participating.
The attenuator can be set to any attenuation value between 0 and 31dB in 1dB steps with a maximum error in any state of about +/- 0.2dB, based on the performance of the prototype. The attenuator was primarily developed for measuring on-air signal to noise levels. The method for doing this is outlined below.
Photographs of the component and groundplane sides of the attenuator is shown below:
Click for larger images
As shown above, the attenuator also includes a 10dB fixed attenuator on the output side. The function of this is to "buffer" the switched attenuator bits from the effects of non 50 ohm loads. Without this fixed pad, errors of up to 1dB have been measured operating the attenuator into a 3:1 VSWR load. With the fixed pad, the errors are reduced to less than 0.2dB.
Each "bit" uses a sub-miniature DPDT toggle switch to route the signal either directly through, or via a "pi" attenuator constructed out of precision 1206 size chip resistors. Standard resistor values were used from the E24 preferred value range, resulting in only very small errors form the ideal attenuations. Care was taken to keep the VSWR of each "bit" as low as possible, to ensure that errors caused by interacting internal VSWRs were kept to a minimum.
The attenuator was designed to operate at 144MHz and below. However, it is capable of useful performance at higher frequencies. One of the G3WDG prototypes is in use at 1269MHz between the up-converter and the PA, to adjust transmit power on the AO-40 uplink. As the frequency increases, however, the 1dB attenuation per step is lost.
In use, measuring signal to noise ratios is very straightforward. The system gain must be sufficient for the S-meter to be reading some finite value, preferably in an area where dB per needle width is as large as possible, probably in the S1 to S3 region. If the system gain is too high, some external attenuation may be needed, and if the system gain is too low some form of amplification may be needed (the attenuator pcb has the provision to add a "modamp" gain block if needed). Off-tune the receiver from the signal to be measured, so that the s-meter is just reading noise, with the attenuator set to 0dB. Then tune in the signal to maximum S-meter reading, and switch in "bits" until the s-meter reads exactly the same as it did with just noise. The number of dB's added is then the signal plus noise to noise ratio. If desired, the signal to noise ratio can be looked up from the table.
The attenuator may also be used for making noise measurements, eg for sun noise, although it has limited use for small noise increases. The initial S-meter reading is taken with the antenna pointed at the sky, and the antenna is then pointed at the object to get maximum noise increase. The attenuator is then used to bring the noise level back to the the original reading, and the dB read off.
Performance data (click images to enlarge) :
Attenuation v frequency
Attenuation error (3:1 VSWR) with no internal pad
Attenuation error (3:1 VSWR) with internal 10dB pad
Draft article for Oscar News (AMSAT UK)