The developers of integrated circuits are no more blessed with the ability to accurately predict the future than anyone else. So although, in theory, a new IC design submitted for production will be fully specified and its characterisation well defined, in practice it is common for additional requirements or specifications to be added after the beginning of the production process.
IC production teams will generally find ways to accommodate late requests from developers. But a particularly troublesome example is the common request to change the way RF filters are characterised by the production test unit. This entails extensive changes to the test software code. This not only holds up the test process and delays the chip’s progress towards tape-out, it also entails the risk of introducing errors into the test code. With the whole production team often under pressure to expedite the fabrication process, the time required for comprehensive debugging of test code is not always available.
We developed a standard test routine that could be used both for characterisation and production at the same time. The routine also needed to be capable of simple modification to make it suitable for a wide range of ICs. Stored in a test code library, this new routine would facilitate the repeated re-use of a base set of test IP.
The chosen method to be investigated was a chirp, sweeping the filter’s frequency range and characterising it in a one-shot measurement.
Figure 1: A chirp
We also considered a multitone to characterise filters. However, this is unsuitable for devices with low input-power sensitivity. To use a multitone, each frequency component has to be divided by the total number of frequencies so that the overall crest factor is below the maximum input power. If this is not done, the individual frequencies interact and cause intermodulation distortion products, which can distort the results in the frequency sweep.
How to build a chirp
In a chirp, the instantaneous frequency of the signal linearly increases with no frequency jumps (see Figure 1). To implement a chirp in practice, the test engineer must allow for the limited memory size of an arbitrary waveform generator (AWG) – a discrete form of the chirp must be created. The formula for a linear chirp is defined by the equation f(t) = f0 + kt, where f0 is the starting frequency (at time t = 0), and k is the rate of frequency increase, or chirp rate.
After performing a chirp sweep and capturing the results, the new method also requires a means to extract the key parameter of a filter that the IC’s designers want to characterise: magnitude response, that is, the 3 dB point, 10 dB point and the bandwidth of the filter. Other parameters that are sometimes also requested are phase response and group delay.
To build the chirp signal in DSP, several parameters need to be understood:
- what is the required accuracy of the measurement for the corner frequencies of the bandpass filter?
- which features of the test instrument support the measurement – memory depth, maximum sampling frequency,