SWEPT FREQUENCY MEASUREMENT APPLICATION NOTE
The following paragraphs describe a low cost swept frequency measurement application that can display the amplitude response of a 2 port device as a function of frequency . This configuration can be applied to the 500 to 1000 MHz band by using the VCO1K and RMA3K or the 1 to 2 GHz band by using the VCO2K and RMA3K.
Refer to Figure 1, during the explanation of each component. The first component required to make the Swept frequency measurement application work correctly is a RF sweep generator (1). Mechanical information for (1) is shown in Figures 9 and 10. In this case the sweep generator is composed of a voltage controlled oscillator, VCO2C, and VCO Driver, VCOTC, that can output a linear increase or decrease in frequency as a function of input voltage (SWEEP IN), from a triangle waveform generator (2). Refer to LB-23 by National Semiconductor Corp for additional information on triangle waveform generators. Because most RF devices possess a less than perfect 50 ohm match, a 6 dB attenuator (3) is needed to keep the device under test (8) from pulling the sweep generator off frequency.
To produce a marker, a coupler (7) is needed to transfer a small amount of the sweep generator power to a RMA1K mixer (4), RF port. The marker frequency is set by a CW generator (5) connected to the mixer (4) LO port, and a small amount of this power is coupled (6) to a frequency counter (13) for accurate marker frequency measurement. The CW generator is composed of another VCO2C / VCOTC combination without a sweep input. When the frequency of VCO (1) sweeps past the marker generator (5) frequency, mixer (4) will generate a DC pulse at the mixer IF port which is amplified (A=100) so that a spike is produced on channel 2 of the oscilloscope. For the marker spike shown in Figure 2 below to move in the correct direction when the CW generator is tuned, the EXT TRIG of the oscilloscope needs to be set to trigger on the falling edge of the triangle waveform because the SWEEP IN signal is inverted by the VCO Driver, VCOTC.
The device under test (8) can be any two port device, filter, amplifier, coupler, etc. In the case of an amplifier, an attenuator needs to be added to the amplifier input to keep it from going into gain compression and the detector (9) from being over driven.
The detector (9) is a broad-band Schottky barrier type. A linear gain stage (10) is needed to boost the milli-volt level of the detector output up to a level needed for the Log amp (11) to produces a 1 volt/decade output. The log amp used in this system is described in application note AN 311 by National Semiconductor Corp. One additional 10X linear amplifier stage is needed at the oscilloscope CH 1 input to produce a 1volt/dB display.
Channel 1 of the oscilloscope shows the frequency response of the device under test (8) and Channel 2 shows the amplified mixer IF output which produces a spike when the sweep and marker generator are at the same frequency. Adjustment of the trigger level will move the response to the right or left, and adjusting the oscilloscope time-base will horizontally spread or compress the shape of the response.
Figure 2, DUT Response
Figure 2 shows the frequency response of a band-pass filter with a center frequency of 160 MHz (nominal), and a bandwidth of 10 MHz (nominal). Cursor 1 = 0.8 dBm, and Cursor 2 = -16.8 dBm. Total range = Delta = 17.6 dB.
Figure 3, Sweep In
Figure 3 shows the basic 10.2 volt P-P, 1 KHz triangle waveform (2) driving the sweep input port on the RF Sweep Generator (1).
Figure 4, Triangle Waveform at the Sweep Input port of the RF Sweep Generator
Figure 4 gives more information about the drive level at the Sweep Input port of the RF Sweep Generator. Cursor 1 = 4 volts which corresponds to 116.27 MHz, and Cursor 2 = 13.8 volts which corresponds to 193.3 MHz.
Figure 5 shows a symmetrical voltage swing around the DC level = Cursor 1 = 9 volts that is set by the RF Sweep Generator front panel tuning control. At 9 volts, the sweep generator output is 154.5 MHz. This level should be calculated to eliminate the possibility of exceeding the upper or lower limit of the VCO controller circuit. The correct value in this case is 13.8 - 4 = 8.9 volts. In the case of a narrow band device the center frequency is set first by the RF Sweep Generator front panel control and then the sweep amplitude is increased to include the DUT side-bands.
This next section deals with how to measure the 3 dB bandwidth of a device under test. Cursor 2 is set to intersect the peak of the curve as shown in Figure 6. The marker oscillator is set so that the spike also intersects the peak of the curve, and the frequency displayed on the frequency counter is the center frequency (Fc) of the DUT. For this DUT, Fc = 162.0 MHz. Then, cursor 1 is moved to a point where Delta = 3.00V. Since the Calibration is 1volt/dB, this is the point where the DUT response is down by 3 dB. Bandwidth is always measured at the -3 dB points. Next as shown in Figure 7, the marker is moved to the right to measure F2. In this case, F2 = 167.5 MHz. And finally, the marker is moved to the left side as shown in Figure 8. F1 = 156.5MHz. The bandwidth is calculated from this information, BW = F2 - F1 = 11 MHz. To check the accuracy of this test, the following is applied, F1 + BW/2 = F2 - BW/2 = 162.0 MHz = Fc.
Figure 6, Center Frequency Peak
Figure 7, -3 dB point for F2
Figure 8, -3 dB point for F1
Figure 9, Sweep Generator Front (see Figure 1, Item 1 for circuit information)
Figure 10, Sweep Generator Back (see Figure 1, Item 1 for circuit information)
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