Smoke test
Connect the coaxial cables from the output of the RX10700 unit to the RX2500 unit and make sure that the RX2500 is connected to the computer and to the power supply. Do not connect the d-sub connector to the RX10700 unless you are sure the box is properly connected to the computer chassis and the power supply zero. The parallel port interface is an open drain, a 74HC03, and it may be damaged by static electricity. Making sure that ground is connected before the cable to a computer parallel port is connected is always a good idea. Replacing the 74HC03 which is mounted in a socket is trivial, but if the computer parallel port is damaged, repair is not easy.Connect the 9-pin d-sub connector to the RX10700 unit to supply DC voltages and connection to the computer parallel port. Look here for details: Controlling WSE units from within Linrad
In case the current differs much from the nominal 0.6 or 0.7 A something is wrong. There is not yet any experience on faulty boards so I can not give detailed hints for troubleshooting. The currents through the RF and IF amplifiers are conveniently checked by the voltage across the 22 ohm resistors in their power supply lines. It may be a good idea to have current limited power supplies to avoid burning the 1A fuses. In case some error causes an RF mosfet to have near zero resistance, the power in the 22 ohm resistors could become 8W and since these resistors are rated 3W only, they may become damaged if the fail condition lasts for many minutes.
Tuning the crystal oscillator
Set C26 for maximum capacitance to make sure the drain circuit of Q5 is not in resonance. This makes it easier to monitor the Q5 source voltage.If the board is new and has not been tuned before, set C2, C6, C14, C25 and C40 for maximum capacitance. Start Linrad and enter the weak signal CW mode by pressing A. In case Linrad was already running, you have to exit and re-enter by pressing X, then B. This way you clock data into the 74HC174 flip-flops and send current to one of the oscillator transistors. Set the frequency in the Linrad frequency control window to 10.7 MHz. The voltage on R5 (and C1) should now be -4.5 V.
Monitor the voltage on the source of Q5 by connecting a voltmeter to R31. When the LO is not oscillating, the voltage on the source of Q5 is about +0.6V with respect to ground. Tune C24. When oscillations start, the voltage will grow. Set C24 for maximum, then tune C40 for maximum and finally tune C24 again. The source should be somewhere around +1 V with respect to ground now.
While still monitoring the Q5 source voltage, now tune C26 for minimum. Q5 is a buffer amplifier, when the drain comes into resonance the transistor will saturate and the current decrease. The Q5 source should be at about +0.5 V with respect to ground when C26 is tuned for minimum.
The crystal frequencies are 52.7, 52.8, 52.9 and 53.0 MHz. In case you are coarse tuning the board outside its box, you may use a frequency counter on pin 11 of U8 to set the frequencies about 200 Hz above the nominal frequencies. You may also use a frequency counter on pin 8 of U11 to set the frequencies 50 Hz above the nominal LO frequencies.
Tune X-tal LO
Cold Warm Cold Warm
C2 52.7002 52.700 13.17505 13.175
C6 52.8002 52.800 13.20005 13.200
C14 52.9002 52.900 13.22505 13.225
C25 53.0002 53.000 13.25005 13.250
The temperature coefficient of the crystals is not equal.
Tuning cold for a frequency error of 50Hz will typically
lead to frequency errors within +/- 50 Hz for the warm unit.
Coarse tuning of the RF filters
Connect a signal generator to both RF inputs with a T-connector and 2 10 dB attenuators. Set the frequency to a few kHz from 10.7 MHz. Anything between 1kHz and 25 khz will be fine. A suitable power level is -20dBm, but the level is not critical.Select I=SOUNDCARD TEST MODE on the main menu to display the audio signal with the oscilloscope mode. (In early versions of Linrad this function was called F=HARDWARE TEST MODE.) The keys "1" and "2" are used to select the channel and "+" and "-" can be used to set the oscilloscope gain. Note that the crystal has to be selected before the test mode is entered. If you follow the procedure outlined above it will be automatic, the crystal is selected by setting the frequency to 10.7 in the A=WEAK SIGNAL CW mode.
Tune C92, C93 and C100 for maximum signal in channel 1 and C51, C61 and C62 for maximum signal in channel 2.
Fine tuning of the RF filters
In case you do the coarse tuning very carefully, fine tuning is not really necessary. You can also skip the coarse tuning and go directly to the fine tuning procedure.The fine tuning procedure uses a pulse generator. Set the pulse repetition frequency to something like 200 Hz.
To have the pulses injected into both channels simultaneously, use a hybrid followed by two 10 dB attenuators. The hybrid will isolate one input from the other only if the output impedance of the pulse generator is 50 ohms. Without the attenuators, the tuning of one channel will affect the other channel if the pulse generator is a parallelled 74AC74 or similar. The filter tuning has some influence on the input impedance.
Connect the RX10700, the RX2500 and the computer as described above and enter the J=ANALOG HARDWARE TUNE mode. Then select RX10700. With a correctly tuned RX10700 unit you should see two spectra side by side as shown in fig. 1.
Fig.1. The screen in ANALOG HARDWARE TUNE mode. The distance between the horizontal lines is 1 dB. The spectra can be moved up or down with the '+' and the '-' keys. This unit is correctly tuned.
The ANALOG HARDWARE TUNE mode measures the four spectra that can be obtained with the RX10700, combines them to a spectrum that is about 160 kHz wide (90 kHz + 3 x 25 kHz). It is easy to tune the filters while looking at these spectra.
The RX2500 has to be calibrated and the fft1 bandwidth must be set to at least 100Hz.
Another way to tune the RX10700 RF filters is to use a network analyzer in conversion loss mode. Then tune for a flat response from 10.630 to 10.795 MHz. In this case it makes no difference which crystal has been selected.
Checking gain and intermodulation
Connect two signals, both with a power level of +10dBm to one of the RX10700 inputs. Place the signals 5kHz apart at for example 10.715 and 10.720 MHz. Make sure the IM3 level of this test signal is below -60 dB regardless of the load impedance.Find out what attenuators you need to make the Linrad S-meter show 110 dB for a 10 dBm signal sent into the RX2500 at for example 2.495 MHz. The RX2500 should be calibrated so the frequency does not matter, it just can not be exactly 2.500 MHz. The attenuator needed is typically 40 dB. You may fine tune with the "First FFT amplitude" parameter. The level seen by the Delta44 will be about 13 dB below saturation and IM3 generated within the RX2500 and within the Delta44 will not be visible. Once you know that a 10 dBm signal sent through the attenuator corresponds to 110 dB on the Linrad S-meter you can use Linrad as a calibrated spectrum analyzer to measure signal levels. Just subtract 100 from the S-meter reading to get dBm. The first production run, 25 units were tested like this with the results listed in table 1.
Unit Channel 1 Channel 2
S(dBm) IM3(dBm) S(dBm) IM3(dBm)
1 10.7 -43.2 10.9 -42.6
2 10.9 -43.7 11.1 -41.1
3 10.8 -41.7 10.8 -43.1
4 10.7 -43.1 10.7 -42.6
5 10.8 -42.8 10.9 -42.8
6 11.0 -43.0 10.9 -42.2
7 11.1 -42.2 10.9 -41.8
8 10.6 -43.5 10.7 -42.3
9 10.7 -43.0 10.4 -43.3
10 10.8 -42.3 10.7 -41.6
11 10.7 -43.1 10.7 -43.1
12 10.6 -43.1 10.7 -42.5
13 10.6 -44.0 10.8 -42.4
14 10.7 -43.3 10.7 -42.6
15 10.8 -43.3 10.7 -42.1
16 10.5 -43.5 10.7 -42.1
17 10.8 -43.3 10.9 -41.6
18 10.8 -43.4 10.9 -42.1
19 11.0 -42.3 11.0 -41.6
20 10.7 -46.7 10.8 -42.6
21 10.5 -43.9 10.5 -43.4
22 10.6 -42.7 10.6 -41.8
23 10.6 -42.7 11.0 -42.4
24 10.6 -43.6 10.7 -43.0
25 10.5 -47.9 10.9 -43.1
Table 1Signal levels and third order intermodulation
levels for the first 25 RX10700 units when two test signals
of +10 dBm each are sent into the unit.
Table 1 shows that the gain of the RX10700 unit is 0.8 dB plus/minus 0.4 dB. The third order intermodulation is at -41 dBm or below so the input IP3 of the RX10700 unit is +36 dBm or a little more. One of the units had IM3 at -39.8 dBm initially. Replacing the input MOS-FET transistor made this unit normal with IM3 at -42.1 dBm.
Checking the noise floor
For this test, use some receiver with low noise figure that can measure the noise levels at the output connectors of the RX10700 unit.The Delta 44 is normally operated in is lowest sensitivity mode ("+4dB" with ossmix) to minimize the noise contribution from the internal amplifier in the A/D converter. To check the noise floor of the RX10700, by use of RX2500 and a Linrad system, set the Delta 44 gain to "-10dB". This way the system noise figure at the input of the RX2500 becomes 9 dB, low enough to see modest deviations from normal noise levels at the output of the RX10700.
To get an even better visibility for the noise generated by the RX10700 a wideband amplifier with a BFR91A was inserted in front of the RX2500 to give a noise figure of 2.5 dB when testing the first 25 units.
Table 2 shows how much the noise floor increases when the output of the 25 first RX10700 units are connected to an amplifier with a system noise figure of 2.5 dB.
Unit Channel 1 Channel 2
N(dB) G(dB) N-G(dB) N(dB) G(dB) N-G(dB)
1 5.2 0.7 4.5 5.5 0.9 4.6
2 5.1 0.9 4.2 6.0 1.1 4.9
3 5.0 0.8 4.2 5.6 0.8 4.8
4 5.3 0.7 4.6 5.0 0.7 4.3
5 5.4 0.8 4.6 5.3 0.9 4.4
6 5.4 1.0 4.4 5.4 0.9 4.5
7 5.8 1.1 4.7 6.0 0.9 5.1
8 5.4 0.6 4.8 5.7 0.7 5.0
9 5.0 0.7 4.3 5.1 0.4 4.7
10 5.6 0.8 4.8 5.7 0.7 5.0
11 5.3 0.7 4.6 5.5 0.7 4.8
12 4.9 0.6 4.3 5.3 0.7 4.6
13 5.5 0.6 4.9 5.2 0.8 4.4
14 5.1 0.7 4.4 5.4 0.7 4.7
15 5.3 0.8 4.5 5.7 0.7 5.0
16 5.0 0.5 4.5 5.6 0.7 4.9
17 5.2 0.8 4.4 6.1 0.9 5.2
18 5.1 0.8 4.3 5.4 0.9 4.5
19 4.8 1.0 3.8 6.2 1.0 5.2
20 5.1 0.7 4.4 5.4 0.8 4.6
21 4.7 0.5 4.2 5.3 0.5 4.8
22 4.7 0.6 4.1 5.6 0.6 5.0
23 4.9 0.6 4.3 5.5 1.0 4.5
24 5.5 0.6 4.9 5.2 0.7 4.5
25 4.4 0.5 3.9 5.4 0.9 4.5
Table 2 N is the noise level at the output connector of
the RX10700 measured by comparision with a 50 ohm dummy
load. The test system noise figure is 2.5 dB.
G is the gain taken from table 1.
As can be seen from table 2, the noise floor of the RX10700 is typically 4.7 dB above the noise floor of the measurement system when the gain is accounted for. This means that the system noise figure at the input of the RX10700 is typically 7.2 dB. Taking the contribution from the 2.5 dB noise figure of the measurement system into account one finds that the noise figure of the RX10700 itself is typically 6.5 dB.
Checking close range reciprocal mixing and noise modulation
For this test a low noise crystal oscillator is needed.Inject a test signal at 10.681 MHz, 1 dB below saturation into the channels one by one. The noise floor at a frequency separation of 5 kHz should not increase by more than 2dB in a single channel.
In the first production batch of 25 units, none of the boards failed this test.
The test signal, 1dB below saturation is at 126.7 dB on the Linrad S-meter. The noise floor with a dummy load at the RX10700 input is at 9.7 dB in 1 kHz bandwidth. At a distance of 5 kHz from the test signal, the noise floor is typically at 11.0 dB which is -145.7 dBc/Hz. Most of this noise is generated in the Delta 44. Note added Oct 26 2003: The first 25 units were tesetd at two separate occasions since this batch was delivered in two rounds. The typical 1.3 dB noise floor degradation observed for units 1 to 11 turned out to be 0.8 dB for units 12 to 25. The reason is most probably that I did not set the Linrad parameters properly the first time causing some digital noise to interfere with the measurement. Suitable parameters are:
First fft bandwidth = 10 Hz
First fft window N = 2
Second fft disabled
First mixer bandwidth reduction = 5
Checking medium range reciprocal mixing and noise modulation
For this test a crystal notch filter is needed.Inject a test signal at 10.662 MHz with a level of +5 dBm into the channels one by one via a notch filter at 10.682 MHz. Set Linrad to 10.725 MHz so the strong signal will reach the RX2500 at 2.563 MHz causing an audio frequency of 63 kHz after mixing with 2.5 MHz. Run the Delta 44 in high gain ("-10dB").
The noise floor should not rise by more than 2.5 dB.
The noise floor change is measured at the center of the notch filter, 10.682MHz, which is converted to an audio frequency of 43kHz. This is pretty close to the upper limit of the Delta44 frequency range so the system noise figure at the RX10700 input is about 11.5 dB due to the extra noise of the Delta 44 because of the high audio frequency.
A noise floor increase by 2.5 dB (1.78 times) means that the sideband noise due to the strong signal is 78% of the power associated with the 11.5 dB noise figure, (4097 K). This means that the noise temperature of the sideband noise is below 1802 K or below -163.6 dBm/Hz. Since the test signal level is +5 dBm the test ensures that reciprocal mixing and noise modulation together are below -168 dBc/Hz at a frequency separation of 20 kHz when the strong signal is well outside the visible passband.
In the first production batch of 25 units, none of the boards failed this test.
Checking wide range reciprocal mixing and noise modulation
Inject a test signal at 10.782 MHz with a level of +10 dBm into the channels one by one via a notch filter at 10.682 MHz. Set Linrad to 10.675 MHz so the strong signal will reach the RX2500 at 2.393 MHz, well outside the 2.5 MHz RF filter. Measure the noise floor at 10.682 MHz with and without the test signal. Run the Delta 44 in high gain mode ("-10dB").The noise floor should not rise by more than 3 dB.
This test gives the noise floor at 100 kHz separation for signals outside the visible passband. A noise floor increase by 3 dB means that the sideband noise due to the strong signal is equal in power to the system noise associated with the 10.5 dB noise figure, (3262 K). This means that the noise temperature of the sideband noise is below -163.5 dBm/Hz. Since the test signal level is +10 dBm the test ensures that reciprocal mixing and noise modulation together are below -173 dBc/Hz at a frequency separation of 100 kHz when the strong signal is well outside the visible passband.
In the first production batch of 25 units, none of the boards failed this test.