Getting Started |
Measurement Capabilities
4
2.2
MEASUREMENT CAPABILITIES
The Signal Hound is capable of making a wide range of measurements, with resolution bandwidths from
less than 1 Hz to 250 kHz. The internal I/Q demodulator captures up to 2 Megabytes of information each
second, with a hardware-limited bandwidth of 250 KHz. Sweeps with spans greater than this are actually
a combination of many smaller sweeps, mathematically combined to reject image and spurious
responses.
2.2.1
Image and Spur Rejection in Swept Mode
The USB-SA44B does not have hardware-based image rejection, instead relying on a software algorithm
to reject image responses. The algorithm mixes the incoming RF with two distinct local oscillator
frequencies, typically spaced by 21.4 MHz and up to 100 milliseconds, and rejects responses not present
in both. This algorithm has some limitations:
1.
A signal must be present for both captures to be displayed. Pulsed or swept signals, which do
not stay at any given frequency for this duration, will be rejected as potential image or spurious
responses.
2.
An analog modulation envelope may be clipped, since certain frequency components of the
modulation envelope may not be present at both times. Most digital modulation tends to spread
energy evenly across its bandwidth and is relatively immune from this effect.
3.
Two RF input signals, spaced by 42.8 MHz, will generate a spurious response halfway between
the two RF input signals. This spurious response will not be present when a 200 kHz span is
selected. Broadband signals which exceed 42 MHz cannot be accurately measured with the USB-
SA44B because of this effect.
Disabling image rejection will allow pulsed and swept signals to be displayed, and will not clip
modulation, but image and spurious responses may be a major problem for some measurements. If
your application requires hardware-based image rejection, consider our BB60C.
Wide sweeps of strong signals, especially at frequencies above 1 GHz, may have noticeable fractional-N
spurs. To avoid these spurs, set your span to 200 kHz or less. In this narrowband mode, the incoming
signal is mixed using two distinct local oscillator frequencies with different frac-N modulus settings, to
two distinct intermediate frequencies, such that fractional-N spurs are completely masked out.
2.2.2
Real-Time Mode
The USB-SA44B can continuously stream up to 250 kHz of spectrum to the Spike
TM
software running on
your PC or laptop. Real-time mode displays this stream of data in the frequency domain.
For modulated signals not exceeding 250 kHz bandwidth, real-time mode is recommended, as it will
capture and display the modulation envelope using overlapping FFTs, showing you an accurate
representation of the modulation envelope. All modulation types, including pulse and short digital RF