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MC13142D Folha de dados(PDF) 11 Page - Motorola, Inc |
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MC13142D Folha de dados(HTML) 11 Page - Motorola, Inc |
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11 / 16 page  MC13142 11 MOTOROLA RF/IF DEVICE DATA current sources to the Cell and can be programmed (via Pin 15) for more current. The current is often adjusted for minimum third order response. In this Fixture it is fixed biased for most conversion gain. The Mixer circuit is balanced where both oscillator and RF are suppressed. This provides IF signals at Pins 9 and 10 which are equal in amplitude and 180 degrees out of phase. To realize a positive gain one needs to reflect a higher impedance from the load impedance (50 Ω for this fixture) to the Mixer output or outputs. Maximum signal transfer would require a balance to unbalance network. Center tapped tuned transformers can perform this function but are quite expensive. If one can afford 3.0 dB less signal, a simple LC circuit at one of the outputs will work well. The other output is unused and bypassed to ground. The most gain is realized when no shunt capacity is added and L4 is selected to resonate with the terminal capacity. Adding shunt capacity will lower the gain and increase the circuit’s bandwidth. A small value series capacitor C4 to the 50 Ω output will control the reflected impedance and complete the circuit. L4 and C4 will vary in value depending on the IF frequency. VCO The base of the device is the source for driving both the Gilbert cell and prescaler buffer stages. Because of this, the oscillator device will operate and drive the Mixer only in the grounded collector configuration. Additional dc bias is added through a 1.3 k Ω resistor (tapped for minimum VCO loading) to reduce the off–set between base and supply. The external circuit is a modified Colpitts where the capacitance between base and emitter (Pins 4 and 5), along with a capacitor from emitter to ac ground, forms the circuit capacity and the feedback that sustains oscillations. The effective circuit inductance (looking from the top of the circuit, the transistor base) consist of L3 in series with varactor diode D1 and a blocking capacitor. This circuit must appear inductive for the VCO to operate properly. If the capacity is too small, the feedback ratio is reduced and the VCO can cease oscillating. When it becomes to large, it will not vary the frequency due to the limiting effect of the series loop capacitance. In this application, the VCO is not required to cover a large tuning range. Limiting the tuning range to no more than is required to cover the band (making allowance for temperature and aging effects) will result in a VCO less susceptible to on board noise sources. To assure oscillation while controlling the tuning range the varactor (plus series capacitor) minimum capacity is chosen to be about equal to the capacity from Pin 5 (transistor base) to RF ground. The maximum tuning ratio could be no greater than 1.41 because the circuit capacity could only double whatever the upper value capacity the varactor attained. An upper limit on the varactor capacity along with the effects of the series capacitor reduces the VCO tuning range to about 1.2 times. The varactors chosen for the test fixtures were Loral KV2111. The VCO buffer, as most emitter follower circuits, has the potential of generating a parasitic oscillation. When a collector is RF bypassed, a tuned LC circuit is formed consisting of the bypass capacitor, bond wire plus package pin inductance and the device effective output capacity. If the base is low impedance, there is normally enough distributive collector to emitter capacity for the device to oscillate in the common base mode. A simple fix without affecting the buffer otherwise, is to place a small value series resistor in the collector lead. This will lower the Q of the circuit where it cannot sustain oscillations. Without the series resistor at Pin 8 or some other damping element, the buffer will oscillate. PLL A phase lock loop is added to the test board to evaluate the VCO. The MC12179 multiplies the crystal reference frequency by 256 to obtain lock. In a frequency agile system, the MC12210 would control the VCO and its reference derived from a crystal. The crystal frequency would be selected to coincide with the required VCO frequencies and channels spacing requirements. Expected Performance As stated earlier, the MC13142 performance in any of the systems should mirror the performance obtained in the test fixture. Fixture power gains of 15 dBm and noise figures of 5.5 dB are typical. The Mixer current can be varied to enhances battery life as well as alter its output characteristic for peak performance of a desired or undesired response. |
Nº de peça semelhante - MC13142D |
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Descrição semelhante - MC13142D |
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